What we learn by studying other worlds is useful in helping us protect the Earth as well as the limits of life in hostile environments. Also the international co-operation for space exploration is the sort of co-operation we can make use of on Earth too.
Saving Earth is very important, as we see with climate change. But it’s not either/or — there are the resources to do both. Even NASA’s budget, the largest in the world, is maybe 0.5% of the US budget. The numbers are similar for other countries, so there is the ability to look up at the sky and look down at the Earth and solve problems in both places. In fact, answers in one place might help solve problems in others — the greenhouse effect seen on Venus helped us think about global warming on Earth. To me, it’s win/win. I do completely agree that we don’t want to make a mess of another planet, so we need to learn our lessons from this one.