Energy: unlimited solar power, exotic helium isotopes for future fusion reactors.
Materials: metals are abundant everywhere in the solar system. Metal is getting more and more expensive to mine and recycle here. There will come a tipping point when it is cheaper to get it from the moon or asteroids.
Room. We are going to need it. There's a spare planet next door named Mars that could be tweaked to make a second earth. It already has water. So does the moon.
Like I said, this is only important if we intend to keep developing new technology, making more people and expanding our horizons. If we are going to go back to hunting and gathering in small bands it isn't necessary. And that is the only other choice. We can't live like we live now forever. Our choices are up or down.