I can think of a few reasons. First, because personalized instruction and discussion makes a big difference in actually gaining real understanding. Second, beca

I can think of a few reasons. First, because personalized instruction and discussion makes a big difference in actually gaining real understanding. Second, because most people won't push themselves as hard on their own. Third, because having someone with personal knowledge of you and your capabilities means something, especially with online things when cheating is trivial.