More specifically, when you go to an event that is ITC you will be asked for your name and email address. The email address is basically your login/registration into ITC. If you want to participate in ITC you have to provide an email.
If the event uses Best Coast Pairings (BCP) when you sign in or buy your ticket through BCP you will give your email and put in your info for the first time. From then on that email will auto populate the names field and club anytime you go to a BCP event in the future. At the end of the event results are uploaded to BCP and now your points are tracked for the future.
Some events don't use BCP, those will still need your email.