Terraforming is not easily viable on Mars. (as mentioned, atmosphere would be rapidly stripped away, FA protection from radiation)
However, people could exist in "mars bases" - ideally underground as much as possible for less grief from storms & temperature changes.
Also has advantage of more gravity than the moon (though only about 40% Earth - so might lead to a few nasty health consequences in long term, or conversely might have benefits as lower gravity being less hassle on the body - who knows until we try it & do the research).
So, given there is anon zero possibility of sociopath politicians (or just incompetence / accident) leading to us trashing Earth so its not readily habitable by people for a while (be that a long time, or just centuries) then worth a few self sustaining colonies (and seedbanks etc.) as an insurance scheme on Mars, just in case (though cynic in me does think might be better if we did wipe ourselves out & give evolution a chance to try some new stuff)