Well, there's a kind of reverse way of looking at this, but it doesn't fit our Western ideas very well. But explanations based on Western ideas, they can sometimes kind of miss the main point in the practice.
ENCOURAGING GENEROSITY AS A MONK
The way Buddhists look at it, then you help other people by giving them an opportunity to show generosity. Because being generous helps the person who is generous.
We tend to think that if you give something to someone, then you are losing something, whatever it was you gave them - and they are gaining something.
But with this idea, then you both are benefiting, the person who receives, and the person also who gives.
If you think about it, it is a very kind of limited way of looking at things, to say that the only person who benefits in that situation is the one who receives. As if everything could be measured in materialist terms.
There are many stories about miserly people who become more and more unhappy and closed in on themselves, and poor people who are happy. Of course can be the other way too, wealthy people who are open and generous and poor people who are miserly and live a rather miserable life.
And, miserly people may seem to lead happy lives for a while. But - in their apparently successful and happy lives, they are missing something, are kind of a bit locked in on themselves, not even realizing that they are.
So - the Buddhist monks, by being homeless, and dependent on others for the very food they need to survive, are giving others the opportunity to be generous by supporting them.
DEPENDENCE OF THE MONKS AND NUNS ON THEIR SOCIETY
Of course it's not possible for everyone to be a monk or a nun, that would never work :). So, the monks themselves (in what follows I will say monk meaning monk or nun) are also dependent on their supporters. So it's a two-way thing. It also means that every day the monks are acutely aware of how they depend on others. We are all dependent on others, but monks - especially if they practice in the traditional way where they are dependent on begging for all their food - then they are acutely aware of how dependent they are on everyone else.
Which helps them to have true humility, amongst other things. A bit difficult to be proud of your standing as a monk, when you know that if you aren't given food that day, you will go hungry (in the traditional rules of a monk, then you are not permitted to keep any food from the previous day over to the next day, so you are totally dependent on what you are given that day).
Nowadays, only some monks practice like this, and many live in monasteries, and their food is provided to them by the monasteries. In the mahayana traditions, then the vows are not observed in such a strict way - in the original vows monks are not permitted to handle money, but many do in the mahayana tradition, for instance. They are not permitted to eat after midday, but in the mahayana traditions many do.
But - this is understood through the distinction between major rules that you have to keep (if you are fully ordained monk that is), and minor rules that are not essential. Buddha - who laid out the monks vows originally - said that there are these two kinds of rules, but nobody thought to ask him which were which. So nobody knows, and the Therevadhan schools generally play it safe and treat it as if all the rules were minor ones, the Mahayana ones tend to be more selective and of course celibacy, not killing, not stealing etc are major rules, but tend to treat other rules, especially ones that make it tricky to interact in modern society, as minor rules, out of compassion for the people in the society they live in.
Still the basic point is there, that the monks are dependent on the others in their society to survive, and it is something that if practicing properly, they will be acutely aware of throughout their practice. As we all can be, but their rules and guidelines and meditations, and so on are meant to make this very clear to them, so they can't miss it.
There is more to it though.
OBJECTIVE OF YOUR LIFE
Again some people tend to think that the objective of life is to be as wealthy as possible. We even talk about "how much someone is worth" measured in terms of how much wealth they have.
Of course there are many who don't think that way.
For instance a cook may want to be an excellent cook, or a gardener an excellent gardener, a musician or composer may want to be top of their profession, academics, painters etc, wealth are often not their main objective, or sometimes, not an objective at all.
Sometimes honours aren't their objective either. Or esteem of others, sometimes they just have the aim of excellence in their profession, not caring if anyone else knows. For instance a good doctor may be a good doctor just with the main motivation that this is what his or her patients need.
As an example of how little some care about such things, the mathematician Grigori Perelman who declined the Field's medal, and Jean Paul Sartre and Le Duc Tho who both declined the Nobel prize Nobel Prize Facts.
But - even that is kind of stressful. We can't all be the best cook, or best gardener, or best composer. That again could never work.
BUDDHA'S TEACHING ON THIS
Well - as the Buddha taught, nothing is permanent, it's not possible to solve all your problems by amassing wealth or honours.
These can help in the short term, you may have a very happy life for a while. But if your happiness is dependent on wealth, or the respect of others, or good physical health and athletic prowess or strength, or balance, or whatever it is, you know at some level that it is not permanent. That you are dependent on these qualities, which at some point will no longer be there.
Nor can we find a solution to all our problems in more subtle ways by finding pure states of mind where your mind is calm and peaceful. You can learn to enter such states by mental disciplines - but so long as they depend on training, and conditions, then they don't solve your problems either.
He taught that there is a truth you can come to see, in this very life. Which involves looking directly at what is true, and no longer fooling yourself in the many ways that we do with our confused thinking. A truth you can't learn from books, or by following a chain of reasoning, because anything like that also, obviously, would be dependent on conditions (remembering the chain of reasoning leading to the conclusion for instance).
If there is such a truth, as he taught, clearly it is something you will have to see for yourself, directly, in such a way that it is unmistakable.
And, he demonstrated with his life, it is a truth you can see in this life. You don't have to die first to see it.
So, when the monks and nuns lead a homeless life, they are following this path - to find a truth, a path, that doesn't involve amassing wealth, or honours, or being clever, or stupid, but just following this path of the Buddha, directly relating to things as they are.
They do this following a path that is non violent - not harming anyone, at its basis, and based on telling the truth wherever possible, not taking things that are not yours, without deceit.
Then based on that foundation, opening out to all the situations you are in and to others, and the situations they are in also. And then, on that foundation, of opening fully to your situation, without boundaries, you then find the stability and confidence to be able to relate directly to things as they are, in yourself and in others, no longer hiding behind confused thought.
So, of course you don't need to live the life of a monk or nun to do that. But - they are like the outward, easy to see signs of this path. They are like an outward clear sign of the first of those foundations - the non harming and truthful and honest basis. So by giving to the monks and nuns, supporting them in their lives, ordinary people make this connection to those qualities. And also to the path itself, to the openness of the path, and to the possibility of final realization.
In a way you are connecting also back to the time of the Buddha, when he himself entered into the homeless life, was dependent, as these monks are, entirely on the generosity of others, and finally reached enlightenment under the bodhi tree.
It is kind of symbolic. Of course monks and nuns aren't perfect, make many mistakes and may be confused, may even be deceitful and so on. But by supporting them, lay people are still making a connection to the pure path leading to enlightenment.
So, when Buddhists give food to the monks, they are connecting to all this.
It's the same also if you are non Buddhist. With the traditional way of doing it anyway, monks with a begging bowl, which is clearest in the Therevadhan traditions, you will be very aware that these monks are leading an ethical life based on non harming, and that they are totally dependent on generosity to survive.
So by supporting them, this brings out generosity in yourself, and also a connection to this idea, that in yourself also, you can lead a life of non harming, truthfulness, openness, and coming to relate to the situation you are in, more directly, without the many confusions that so obscure our lives.
TEACHING
Often the monks will give teachings to the lay people. But a novice monk may not have much to teach in terms of understanding of the teachings. It doesn't matter, you teach if you can when you can. If not, you teach through the humility of your silence. But just living the life of a monk is a kind of teaching in itself.
COMPASSION AND LOVE
There isn't any obligation to do community service in the monk's vows. If you wanted to be a teacher, or a doctor, it would be quite hard to do, if you have to get up every day and go around on your begging rounds to collect food. In the Mahayana traditions, the vows are more flexible
But Buddha didn't include anything in the vows to require this. There aren't any rules to enforce love and compassion. In Buddhist teachings, these aren't really things that you can quantify or try to enforce. Compassion springs from the open heart.
In all the traditions of Buddhism, openness, loving kindness and compassion are central to the teachings. Not just in the so called Mahayana schools, in the Therevadhan ones also.
But the vows the monks take are just the foundation for this openness. Which then comes out of their commitment to directly working with openness and truth.
Many monks will take the bodhisattva vows in Mahayana traditions, I think generally all of them would do.
But those aren't like the monks and nuns vows, because, when motivated by compassion, and love, you can't tie it down in the same way, your compassionate heart may lead you to break almost any vow you might try to devise in special situations.
As you open to the compassionate heart, it can lead you anywhere. Sometimes it leads monks and nuns to hand back their robes, out of compassion, to engage more directly in society. You can do this at any time, it is made very easy to do. A monk or nun just has to say to someone of sound mind, able to understand what they are saying, that they are handing back their robes -and that is it done. It doesn't have to be their preceptor or even a Buddhist.
Trungpa Rinpoche is an example of someone who gave up his monks vows. But others may feel they help society better by continuing as a monk or a nun. You can't say at that point what will happen. It's a path and the monk's vows are to do with helping them keep on the path.
And anyone can be a monk, it doesn't matter what you've done. Could be a criminal, a mass murderer. thief, con man, or whatever. And you aren't expected to transform instantly into some kind of a saint because you are a monk. Lay people support them and respect them in Buddhist countries because of the path they are following - they don't expect them to be instant saints however.
That's the basic idea anyway. Maybe others can explain it better, just touching on a few points there.
I wanted to share a few points that I thought might be of interest because of the way they depart from Western expectations and can cause confusion. But no way can I give a comprehensive answer,. please don't expect too much from my answer here :).