An online account 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.
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.
And you can 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!