There are a few reasons. You could say that putting humans on Mars has intrinsic value. In other words, "because it's cool". The other main argument for putting

There are a few reasons. You could say that putting humans on Mars has intrinsic value. In other words, "because it's cool". The other main argument for putting humans on Mars is that the grand projects of humanity push our technology forward. Putting humans on Mars requires solving a larger class of problems than sending probes does. The first space race accelerated the development of rocketry, integrated circuits, computers, satellite communications, and materials science. There's reason to believe that sending humans to Mars will cause similar advances in our capabilities, possibly in novel ways that we can't anticipate. A third argument is existential risk. If something catastrophic happens on Earth, there is some probability that having independent colonies on other planets would prevent extinction of the species. This is definitely true in the long term if we truly master the technology of surviving and thriving in space, but it's less realistic in the near term. That said, I don't think Musk and others working on this problem are taking the challenges of creating a colony on Mars seriously. We know very little about the long-term effects on the human body, reproductive cycle, and psychology. If they're serious about this, one of their top priorities should be funding research to create isolated self-sustaining human colonies closer to Earth as a proof of concept. If we can't survive for decades at the bottom of the ocean or on the moon, we probably can't survive for decades on Mars. We would also need high confidence that generations of humans born on Mars wouldn't accumulate severe genetic damage and birth defects. This is a whole research program in itself that we're nowhere close to solving.