Being registared allows you to store your address and payment details, allowing you to book tickets a lot quicker then it does if you don't have one.
having an account is also great for helping you to keep track of your orders. Log in any time to print a receipt, check your seat details and see if your tickets have been sent yet.
You can also sign up to get email alerts when your favourite bands announce a tour or a show's coming to a theatre near you, so there's no excuse not to keep up to date!