[mr_overalls]

Not to mention that Mars gives humanity an opportunity to improve skills essential to expanding to other solar systems.

Besides the obvious - large-scale terraforming - a "practice" settlement of Mars gives us an opportunity to innovate and refine practically every engineering skill, form of social organization, or general skillful endeavor in humanity's repertoire.

Metallurgy, genetics, geology, farming, medicine, psychology - all of these fields are bound to discover new phenomenon and methodologies under the constraints and conditions of an alien planet.

Not to be glib, but necessity is mother of invention. And there is no necessity as powerful as the drive for survival.

[WorldMaker]

There's an old sci-fi idea, that if I recall Carl Sagan agreed with, that we should be working to colonize Venus now (which I believe "now" meant the 1970s at the time), because worst case if the greenhouse effect spirals out of control unstoppably (as climate change predictions have feared since the 1970s), surviving on Earth is going to be the same problem as colonizing Venus.