I am working a hard science fiction story and I cannot come up with a really good reason to put people on Mars. I need people on Mars for the story to work, but there is no reason for them to be there. Drones can do anything people can (often better than people can). Why should they be there in the first place?

One possible solution I've come up with is maintenance. It's more economically viable to hire technicians to maintain the robotics than build robots to repair the robots.

Edit: I'm looking for city-size populations. These are not explorers (although in an early draft they are prospectors, kinda mining explorers). They could be a support city larger solar exploration.

So I'm going to be an asshole here and poke holes in a few responses on why it won't work in my world. Although some might work well enough to justify changing the world in a way that makes them work (adding social or economic pressures that justify it)

Food exports are really just carbon exports. Any self sustaining colony should have its own "farm" (algae panels like solar panels). They will however need carbon to match their population growth. Earth will never need to import carbon (at least not for a very long time), but a Martian farm could supply extra solar colonies that lack carbon. Lunar colonies would need to import 100% of their carbon; maybe breaking mars orbit an a transfer orbit is cheaper than taking Earth carbon?

A lot of responses don't put humans on the planet. No reason to leave orbit if all you're doing is refueling. And even a refueling station could be pretty automated.

Population pressure represents a new set of problems. Now you have to cheaply move living people. That creates a whole new set of problems. Refugees built a Mayflower to escape WW3? Getting pretty fantastical with that one...

I'm trying to do this with very hard science based fiction, but maybe I have to just assume some speculative technologies work (em-drive, for example) and redesign the economic structure with that in mind. Or create a Unobtainable that puts people on Mars.