- Colonizing space.
- Making better use of our land for sustainable agriculture to feed the hungry and the children of our future.
"Because it's there"
There are plenty of possibilities for extinction events right here. Super-volcano eruptions, radical climate change, polar shifting, airborne super-bugs (hello zombie apocalypse!), nuclear mayhem, etc.
Our local environment also has plenty of hazards. Asteroid impact, engulfment in a large solar flare, orbital shifts are the ones that spring to mind.
Our little niche in the galaxy might also have to deal with wandering black holes, gamma-ray bursts from a supernova that is just near enough, the long-term problem of the collision with Andromeda, etc. We won't overcome any of those by colonizing Mars, but still some things to keep in mind.
On a more practical note, extreme endeavors like colonizing space require some radical thinking (and re-thinking of existing concepts). Colonizing means establishing a foothold outside Earth that is self-sustaining (i.e. can do without a lifeline to Earth). We can't actually do that right now. If we set that as a goal we're going to come up with some very interesting stuff, a lot of which is bound to be useful down here.
Life expands or dies, that's the way it works over here. What the universe really seems to abhor (apart from vacuum), is an equilibrium.
Not only that, but the skills and knowledge we gain with it make us even harder to kill. One of these days we may even surpass cockroaches.
Boosting those boosts economics, creates workplaces and moves towards a better future. Even if they don't invent a hyperdrive or any other way to outsmart spacetime.
Another good thing about it for the countries is that it boosts morale, gathers everyone under national colours etc..
Comparative advantage dictates that some massive chunk of land is devoted to cotton; that way when one of our shirts gets a loose thread we can throw it away as fast as possible so that we aren't seen by others as a leper.