Deadlines are used as a tool to determine whether there is enough interest in a class/activity to invest time and money. If a class minimum isn't met by the deadline, the activity may be canceled. In fact, the quickest way to kill a program is to wait until after the deadline to try to register!
Does missing the deadline mean you can't get into the class? In some cases, participants will still be able to register for an activity until the class is filled. Missing a deadline doesn't always mean you have to miss the activity.
But early registration guarantees you and your family members a spot in your favorite activities, as some classes fill before the deadline.