Well, its mostly to prevent someone from becoming stagnant or gain writers block from not being able to think of something to put, and it will give them time to think of things for future posts. Considering a week is 10 days instead of 7, it will probably get a bit hard to come up with things to discuss not only for classes, but for day to day things. I'd would only do the time skip if I start feeling like things are slowing down and its harder to come up with something to put in for progression.

It wouldn't happen often, but I have come to some points in a roleplay, where I'd ask whoever I'm roleplaying with if they'd like to do a time skip to get to the next part of the story. Mostly if I don't know what to put to help progress.