Managing sign-in credentials has become a complex process. In the beginning, we had just password complexity (e.g. minimum length, at least one number, at least one letter) to deal with. Nowadays, a robust sign-in process also has to deal with password history (don't re-use passwords), account lockout (after a number of unsuccessful attempts), password recovery (security questions), and most recently, multi-factor authentication (using your mobile phone to retrieve a secondary passcode). Rather than dealing with this complexity, it is far simpler to delegate the sign-in process to a 3rd-party service. That's where OAuth comes in. It provides an open standard for performing that delegation.