I'm all in favour of space travel. But purely from a scientific standpoint, what can be done by sending people to Mars that can't be done by sending probes?
EDIT: To clarify (as in the comments), I meant what scientific tests/etc. do we want to do now that we can't with current probe/rover technology, but that we could feasibly do if we sent a person there. I'm not entirely sure if this is too broad or not.