To survive and thrive.
Why build space settlements? Why do weeds grow through cracks in sidewalks? Why did life crawl out of the oceans and colonize land? Because living things want to grow and expand, to thrive, not simply exist. We have the ability to live in space (see the bibliography), therefore we will -- but not this fiscal year.
A key advantage of space settlements is the ability to build new land, rather than take it from someone else. This allows a thriving, expansive civilization without war or destruction of Earth's biosphere. The asteroids alone provide enough material to make new orbital land hundreds of times greater than the surface of the Earth, divided into millions of settlements. This land can easily support trillions of people.
SurviveSomeday the Earth will become uninhabitable. Before then humanity must move off the planet or become extinct. One potential near term disaster is collision with a large comet or asteroid. We don't know where the next killer comet is and although we know where most of the potential killer asteroids are, some have not yet been found. Such a collision could kill billions of people. Large collisions have occurred in the past, destroying many species. Without intervention, future collisions are inevitable, although we don't know when. Note that in July 1994, the cometShoemaker-Levy 9 (1993e) hit Jupiter
If there were a major collision today, not only would billions of people die, but recovery would be difficult since everyone would be affected. If an extensive branch of our civilization is in space before the next collision, the unaffected space settlements can provide aid, much as we offer help when disaster strikes another part of the world.
Building space settlements will require a great deal of material. If NEOs are used, then any asteroids heading for Earth can simply be torn apart to supply materials for building settlements and saving Earth at the same time.
Power and WealthThose that settle space will control vast lands, enormous amounts of electrical power, and nearly unlimited materials. The societies that develop these resources will create wealth beyond our wildest imagination and wield power -- hopefully for good rather than for ill.
In the past, societies which have grown by colonization have gained wealth and power at the expense of those who were subjugated. Unlike previous settlement programs, space settlement will build new land, not steal it from the locals as there simply are no locals. Thus, the power and wealth born of space settlement will not come at the expense of others, but rather represent the fruits of great labors.
A Nice Place to Live
There will be little enthusiasm for moving into space unless people want to live there. In that vein, a few features of orbital real estate are worth mentioning: