Spread the risk and reduce the probability of extinction.

We know for a fact that earth is doomed, on top of our own continuous efforts to kill ourselves off. No not recent climate change type of doomed, but the evolution of our sun is continuously pushing the habitable zone outwards. We might be able to deal with that particular annoyance by hiding underground when it becomes an emergency in half a billion years or so, but our utopia won't be as utopic anymore.

Eventually however, the sun will balloon to a red giant at which point we better have a plan in place other than staying on this planet.

If we're thinking that far out we might as well all just lay down and wait for rain because there's no avoiding the heat death of the universe. Treating the sun dying out like it's a real concern that we need to address in the next 2, 200, 2,000, or 2,000,000 years is comical. Whatever is around to experience that won't be human as we know it.