Sometimes they can mess up your entire day. You'll plan your schedule so that you have most clients and classes on one day and you're usually so busy that every minute counts. So when someone is late, it knocks your whole schedule off course because they'll probably demand that you still give them the full hour - meaning that you'll be late for your next client. However, that isn't as bad as turning people away because you're fully booked to have someone not even tell you that they're not going to be able to make it.
