To me the existential risk argument is good enough. We'll probably want generation ships too, to live without a planet, but Mars is close enough we can get going on some terraforming ideas and within a short timespan create a real last, best hope for humanity should one of several disasters befall Earth.
Here's another benefit: "I don't want to live on this planet anymore" people can leave. And pioneer-spirited in general. Before NA was so colonized, the US used to have a doctrine that if you went out West far enough and worked a piece of land (alone or with a small community) it was yours. Such opportunities aren't around anymore, everything is claimed. On the larger scale, competition in governance models is probably a good thing, if the colony can be made self-sufficient they can get out from the political thumb of whichever parent nations helped set them up.
Of course if I had to pick one future tech to focus on it'd be brain emulation since besides greater consequences that seems feasible on a shorter time scale than terraforming a planet enough for say a million people to live without a dome / in a crater / underground. Non-ems will still probably want to go to Mars but there'd be less urgency for ems who haven't retired. But crater life seems doable even before ems, and we don't need to pick just one thing.