So far our legacy as a species on earth is one of destruction. Only by taking on the challenge a place that is already dead can we be certain of creating something new. Maybe we will learn some perspective in the process.

Going to Mars will have myriad discoveries & side effects, if we knew what they were what would be the point of going?

Also, what if curing cancer is harder than going to mars? One is by now a fairly quantifiable objective, the other is debugging a mind bogglingly complex system with no version control.