The typical login form offers many options for users to sign in to their account. With so many choices, users may spend a considerable amount of time making a decision. They'll have to consider each sign-in option and determine which one suits them best.
Not only that, but if they accidentally select the wrong sign-in option, they'll have to click the "back" button or close a popup window (third-party apps). This requires extra page loads from the system and more cognitive effort from the user.