Do I have to write course descriptions? Maybe... maybe not. The problem is that you may not know if you need them until you need them RIGHT NOW!
Course descriptions are required by some colleges and optional for others. I think it's probably a good idea to just write them so that you're ready for anything. Others will wait until the bitter end, and then be in a rush when they realize they need it - but it's much harder to do it that way. Sometimes people will call me and say "it's due tomorrow" but it's impossible to come up with course descriptions that quickly! I think it's actually easier to write them as you go. Even when colleges don't "require" course descriptions, it can still help with admission and scholarships.