The first thing I can think of is sample collection. Right now, all the Mars missions have been one-way trips, and there's only so much you can gain from the readouts sent by the rovers. Plus, a crewed mission would be able to bring more varied equipment with them for data collection.
It would also be a major milestone in space travel. Mars is our closest neighbor, and being able to send a crewed mission there makes a whole bevy of other missions more probable.
Why did we send manned flights to the moon when we had already sent rovers there?