Elon has said his ultimate goal with SpaceX is to make humans a multi planet species. This is essential to our survival long term. We KNOW that asteroids large enough to cause a global catastrophe impact the earth on a relatively consistent basis every few dozen million years. We KNOW it will eventually happen again. Even though we are making steps to avoid something like that with DART and other similar missions, the bottom line is, if an asteroid the size of a state is heading to Earth, we have zero chance of doing anything about it. The asteroid that killed the dinosaurs was only a few miles across.

Even if before that happens, there are other things that could cause the end of life on earth; global warming, pandemic, nuclear war, etc. So to hedge our bets, we need to be living on more than 1 planet to survive.

That’s the ultimate end goal. Make it a viable option to get to Mars, and then figure out how to set up a permanent colony there, which eventually will become a larger colony, cities, terraforming, etc. The same way humans have been exploring and settling new frontiers for millennia.