Well, unless we outlaw reproduction without government approval we will hit the limit of what the planet can support as far as human life.

Unless we start recylcing 100% of our materials we will run out of various elements we use for production of effectively everything.

Commercially available sea salt has been found contaminated with microplastics, that means are oceans are already screwed so envetually it would be nice to have a new world to do things right on.

Opening up space opens up more physical space for humans and other life, it opens up more resources for construction (asteroids contain unfathomable amounts of rare and common elements that will be easy to exploit once the appropriate technologies are developed).

Space exploration will continue to add more and more technological and general scientific breakthroughs as it has been now for several decades. Look how GPS has changed the world, it's used in everything from tracking wildlife to your watch to moving goods around the world. Literally impossible if we'd not developed a space program.