If you don't accept
Stephen Hawking's imperative that mankind needs to explore and colonize the stars - because right now we are one planetary catastrophe from extinction - then consider a more mundane yet equally likely reason.
Another reason why we need to explore and colonize is resources. When we run out of things on this planet, gold, copper, and so on, then we would be left picking through garbage to find enough resources to make the things our society uses. It would be expensive but some people claim it's possible to reclaim much so this seems like a less dire scenario.
But there is another more dire twist to this scenario; it is entirely possible that at some point all the resources are used and reclaimed so there is literally no resources left. With population explosion, more people use resources in parallel.
With people living longer and people consuming more products, at some point there won't be any more for those that exist together. Then we have become resource-locked - we can't build anything new unless we demolish what is there first. That would be a massive economic failure. So much resources would go into refabrication the energy cost would waste what we have and jeopardize the future.
So we need to expend resources while we still have flexibility to reach out into the stars, literally so people can all have iPhones and planes.
