Robert Zubrin answers “why should we go to Mars?” in the most eloquent way

The three reasons are 1) it's where the science is, 2) it's where the challenge is, and 3) it's where the future is.

This is further described as 1) if we find fossils, or don't find fossils, then it helps us understand abiogenesis, and if we drill and find life then we can understand biology better, 2) it's a "bracing challenge for our society" that will drive a new generation of scientists, engineers, and doctors, and 3) we need new branches of humanity on Mars, just like the European settlement of the Americas.

He does not (in that short clip) explain why humans need to be on Mars to do the science.