Right now, I think, best to treat it as exploration and discovery rather than colonization. There is plenty of reason to explore - if we discover interestingly different lifeforms on Mars then could be a major new development in biology - if they had a different basis from DNA, or still DNA based but different biochemistry on top (different proteins or metabolism) - or some more primitive form of life now long extinct on Earth - would be the biggest new discovery in biology for a century probably - and endlessly many new results come from that - many surely of economic benefit to pay back costs of the mission.
There isn't much by way of good reasons to colonize especially as far away as Mars at this stage, and a lot that could go wrong if we attempted it, affecting not just the prospective colonists, but future of humanity and things we might want to do on Mars in the future.