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. We have the ability to live in space (see the bibliography), therefore we will -- but not this fiscal year.
The key advantage of space settlements is the ability to build new land, rather than take it from someone else. This allows a huge expansion of humanity 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 colonies. This land can easily support trillions of people.
A Nice Place to Live
A few features of orbital real estate are worth mentioning:
SurvivalSomeday 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. Such a collision could kill billions of people. Large collisions have occured in the past, destroying many species. Future collisions are inevitable, although we don't know when. Note that in July 1994, the comet Shoemaker-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 major space settlements are built 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 colonies and saving Earth at the same time.
Power and Wealth
Those that colonize space will control vast lands, enormous amounts of electrical power, and nearly unlimited material resources. 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 colonization programs, space colonization will build new land, not steal it from the natives. Thus, the power and wealth born of space colonization will not come at the expense of others, but rather represent the fruits of great labors.