From your profile icons, it looks like you have completed it.
Staff here use it to find your account details so they can check your line etc. Without this they have no way of checking your details to look into your issue.
It's no different to when you phone and go through security checks, your profile is the forum security check and stops others trying to hijack your account.
Please remember to mark Solved Posts with Best Answer. Doing so helps other customers and saves TalkTalk's Support Team time by only looking at unsolved topics. Thanks, Steve (a fellow customer).
