I'm vaguely aware of what single sign on is. Though I don't use it, it seemed non-intrusive. The most you would see buttons to login through Google, Facebook etc.

Now some sites have an entire banner pestering you to sign in through your Google account. It has a big continue button that takes you to Google and the button to close is a small x in the corner. On mobile this banner takes up a third of the screen.

Why do this?