I certainly would only trust my textbook as the authoritative source, but i can see that in the absence of an expert teacher it's nice to have something that can critique a proof. Imo the fact that it's hard to verify that your own proof is correct is one of the main barriers in self-studying math, especially if one is at a level where one is not completely fluent in applying the various techniques. This also applies to judging answers to open-ended questions in any other field.