To explore the planet and especially to find out if there is life there. But - best place for them to go is to orbit around Mars or to Deimos or Phobos. There, they can explore the entire surface by telepresence.
And Deimos and Phobos both have many advantages over Mars anyway.
Most important though - if you are searching for life on Mars, it would be the biggest anticlimax in biology if you find life you brought there yourself. The jury is out at the moment about whether there are habitats for present day life on Mars, but many think it is possible, with many suggested habitats there.
Until we know more about that, any human landing risks greatly increasing the chance of introducing Earth life to the planet. Especially if it crash lands, and especially if due to some piloting error it crash lands in the higher altitudes. But even if it crashes in equatorial regions, with the global dust storms, hard to see how the contamination could be contained or reversed, if there are habitats there.
While, in orbit or on Phobos or Deimos - it's a spectacular very interesting mission. Wonderful views of Mars. Close up telepresence of the surface.
And far safer approach - no "seven minutes of terror" - you can approach it at a gentle walking speed, easier and safer than landing on the Moon. If there are any problems with the landing site, back away a few meters, and try somewhere else. And explore Mars in shirt sleeves environment by telepresence with haptic feedback.
If we go there at all, which would be great but not essential as we can also continue to use robots and if we focus on improving bandwidth from Mars to Earth they can explore Mars far faster than they do today.
But if we do send humans - well I think that's the motivation and I think best mission to do at this stage. With what you do next open ended depending on what we discover about Mars.
It is similar to our motivation for exploring Antarctica. There may well also be adventurers and explorers, tourists too eventually, but as in Antarctica, best analogy because Mars is far less hospitable than Antarctica - by that analogy I expect the main focus to be on science.