A couple of reasons I'm in favour of colonising Mars, the Moon, etc. are:

- No life or ecosystems to disrupt; Mars would be ours to build on, dig up, pollute, irradiate, etc. without wiping out any native species. The only thing to spoil would be the views and landscapes, but we'd be the only ones able to appreciate them anyway.

- Not being stuck on this rock. Mars and the Moon are first steps. In a few centuries, maybe the clouds of Venus, tunnels on Mercury, etc. would be viable. Such colonies aren't "backups" in case of disaster on Earth, any more than people live in Europe as a backup in case of disaster in Africa.

Antarctica doesn't fulfil any of my reasons to live on Mars. In fact, Antarctica is the only place on Earth we've agreed to avoid exploiting! If anything, colonising Antarctica would be a backwards move.

To expand on that first reason, being able to live on Mars (and/or other worlds; I'd personally prefer Ceres or Enceladus, or maybe Pluto when we actually have the technology to perform an orbital insertion there) gives us an opportunity to evacuate Earth and let it recover from the injuries it's sustained during the Industrial Age.

In the Kim Stanley Robinson series on Mars (best Sci-Fi series ever, you should read it), they first 100 colonists train in Antarctica, because it's the closest conditions on Earth to Mars (and even then, still quite far off)

MAny astronauts, on their path to being selected as a trainee for NASA will spend a year in Antarctica maintaining telescopes etc

If reduced gravity is the goal, then what would be the advantage of doing things on Mars versus in orbit, or just on the Moon?

The reason that reduced gravity helps is that after processing, you can loft it back into orbit.

You can't manufacture fuel in orbit (it's a vacuum), and you can't manufacture fuel on the Moon (oxygen can be extracted with a lot of heat, but we can't make rockets without a fuel).

But you can manufacture fuel with Martian atmospheric CO2 and frozen water.

Those are all reasons for not manufacturing fuel in orbit, but the person I was replying to was talking about processing materials from other planets. I get the desire of Mars-as-a-gas-station/space-oil-field, but don't see a benefit to manufacturing other things there specifically because of gravity. If we have the ability to ship materials from planet to planet for processing, then I don't see how you get much benefit flying them down to Mars's surface for processing versus just staying in orbit and shipping fuel/whatever you get from Mars up to you, just like with the other planets.

Not planets, asteroids. Will Belters be able to refine and create high tech alloys in zero g? Will they be able to manufacture parts and equipment in zero g, at scale. It may be that all those activities require gravity, or are exceedingly difficult in zero g, we don't know. In the case that those things are hard in zero g, my proposition is that Mars will be desirable place for those activities due to its low gravity and atm making it easy to send back up.