let's say your username is avan.lue. Someone else wants avanlue, and they try to sign up for it. You already own all variations (avan.lue, avanlue, a.v.a.n.l.u.e, etc) so we tell them that avanlue is unavailable. So they try a different username, and register avanlue1.

Later, they forget that they had to add the 1. When they sign up for Facebook, they type in «avanlue» instead of «avanlue1.» Because you own avanlue, you will get their mail.