People don't scale mount everest because there is an macguffin to be found there. They scale it to scale it. Because you can. For the adventure. To assert dominance over the universe.

We can talk about economical benefits or the survival of the species and those do matter. But it's also because colonizing another planet would be fucking epic and go down in history.

With the decline in religion we do need some kind of higher purpose and meaning to rally behind, and going off planet could fit that bill. In a time of increasing divisions it could foster brotherhood.