We don't know what we'll find. Or what we will learn. Or what technology we will develop along the way. Most of science is like that. You're proceeding to the unknown. You cannot know exactly what you will find. Predicting it and judging investment based on a hypothetical hoped for goal is folly.

What you can do is make steady progress. And if we want to go to Mars, or more interestingly, Europa, Titan, Callisto and Enceladus, we need to start with the Moon. Flex long unused muscles. See what we can improve, perhaps set up a moon base for interplantery travel... All that is helpful for our future.

As a species, we seem pathologically incapable of solving global warming. In reaching for other planets, maybe we'll find a way to let a small part of us survive. Seems a good enough reason to try.