When you come to check in at the convention, we still have to enter you in the system that everyone else is entered in. As stated, you don't pick a time slot when you pre-reg...you still have to come to us to pick that and to say, "Hey, I didn't have a last minute change of plans/I didn't flake--I'm here, this is the time I want." You still come to me, hand me your pre-reg stuff, I still have to type it in, you still have to get a call-number, etc.
Contest pre-registration only saves you the hassle of having to hand-write some info that most people had filled out before they got to me anyway