@Kallika- It's to prove that you're you and you're not, say, picking up someone else's badge, or signing up in someone else's name so they'd have a hassle later. Also, of course, to prove that you're over 13 (or whatever the age limit is x-x). Having your parent/guardian with you with their ID usually works too.
It's weird that you weren't checked, but I've always preregged, so maybe they're more lenient with minor registrations now O-o;
It might be better to wait for a staff response, since I obviously have no idea what I'm talking about. ><