At good schools nearly everyone graduates in four years, but at lower level schools lots of students don't finish at all or take more than 4 years in part because they fail (or never finish the work) in classes. Given the importance of getting a degree, and the cost of taking more than 4 years to do so, grading is also important for students "at the bottom" of the college world.
Good point, thank you. I was focusing on the top half of the distribution, when there is also a cutoff in the bottom half.
Another month, another rationality quotes thread. The rules are: