One thing I never appreciated as a student was that, no matter how heinous the work I was assigned in class, the professor had to do even more work than I did. If I'd thought about it at the time, I never would have complained about my workload, ever.
For one, the professor has to actually do the assignment, same as the students. It's much like beta testing -- gotta make sure the assignment actually works, that it isn't too advanced or too time-consuming. It's also useful for having an "A"-level standard to use while grading (assuming you could get an A in the course you're teaching :-).
Of course, before testing the assignment, the professor has to actually design the thing. And afterwards the professor has to grade all of them. And during the assignment period the professor will be fielding questions about the assignment during office hours.
At least, that's what it's looking like to me so far. Maybe I'll discover a shortcut later.