You would also find out pretty quickly why you should have registered when you go to the rectory to arrange your wedding, or baptize your child. Who are you?
Also, when you sponsor a confirmandi, or become a godparent, you have to prove that you are a member in good standing in a parish. You are? Prove it.
Some dioceses also use the number of registrants to assign priests. Low registration = one priest.
Have I convinced you? You probably can register on line.