They get through school faster, we have a greater capacity in the university to serve more students.
If you're taking fewer semester credit hours than would allow you to graduate on time, in other words if you're taking 12, 13 or 14 hours there's going to be a penalty, you're going to have to pay more.
The major reason is that we think it's a real incentive, hence the name, it will get students to graduate on time.