These are my reasons:
(1) It'll motivate you to study. It sets a "due date" of when you know you should finish studying by. I guess you can say it puts on the pressure 🙂
(2) Spots may fill up and/or you may not get to choose the time and date you want if you wait to sign up.
You can always reschedule for a fee of $25 IF you don't feel ready. Just be sure to do it at least 2 days before your test date.