This planet can't protect us forever. Sooner or later, there'll be a catastrophe that renders this world uninhabitable for humans. And when that day comes, we'll need to know already how to live in space.
Physicist Stephen Hawking suggests that our ongoing efforts to colonize space could ultimately save humanity from extinction. As it stands, Earth is our only biosphere — all our eggs are currently in one basket. If something were to happen to either our planet or our civilization, it would be vital to know that we could sustain a colony somewhere else.
And the threats are real. The possibility of an asteroid impact, nuclear war, a nanotechnological disaster, or severe environmental degradation make the need for off-planet habitation extremely urgent. And given our ambitious future prospects, including the potential for ongoing population growth, we may very well have no choice but to leave the cradle.
Back in 2000, NASA completed a $200 million study called the "Roadmap to Settlement" in which they described the potential for a moon-based colony in which habitats could be constructed several feet beneath the lunar surface (or covered within an existing crater) to protect colonists from high-energy cosmic radiation. They also outlined the construction of an onsite nuclear power plant, solar panel arrays, and a number of methods for extracting carbon, silicon, aluminium and other materials from the surface. As NASA's roadmap suggests, a colony on the Moon could help us prepare for a mission to Mars. It would probably be wise to set up, test, and train a self-sustaining colony a little closer to home before we take that massive leap to Mars.
And indeed, Mars holds considerably more potential than the Moon. It features a solar day of 24 hours and 39 minutes, and a surface area 28.4% less than Earth's. The Red Planet also has an axial tilt of 25 degrees (compared to the Earth's 29%) resulting in similar seasonal shifts (though they're twice as long given that Mars's year is 1.88 Earth years). And most importantly, Mars has an existing atmosphere, significant mineral diversity (such as ore and nickel-iron), and water. Actually, it has a lot of water. Recent analysis shows that Mars could have as much water underground as Earth.