Glory, and PR.

Also, if traces of life are found, dead or alive. Not many other reasons, and I'm afraid that it'll cost so much and return so little that it'll kill off manned solar system exploration for good. The moon landings were so one-shot and expensive it hasn't been attempted in 60 years. Why woulda Mars mission be any different?

Proper pacing would be to try some asteroid mining, to get various types of mass into low earth orbit for constructing the next wave of exploration.