Why cross the ocean? Or climb a mountain range? Or cross the Mississippi?
If you don't grow, you die and the human race needs to start growing again.
On a technical side, it's the perfect place to start exploring the solar system from. We live at the bottom of a huge gravity well that makes it extremely expensive and difficult to put things in orbit, let alone to move them around to other places in the system. By the time you get out of Earth gravity well you've used most of your fuel.
If you could jump start a technology base on the moon that could build, or even just fuel ships that never had to land on Earth, it would be far easier to explore the rest of the solar system.
