[cynest]

Often it's required of the prof as well. Mandating classroom attendance is a way for colleges to retain students through two means:

1): Making N% of the grade attendance based means that students have to do even less well to fail the course.

2): For a student who would need to learn the material to pass, there is a learning benefit to attending the class. Moreover, more people in a lecture hall is going to create an impression that you do need to show up to pass.