It is an alternative login system separate from Facebook. It allows users the option to bind their current game account to Google. However, the binding option can only be used when there are binding attempts still remaining.
Your account can only be bound to one service (Google ID or Facebook) at a time. Changing to the alternate binding option will require you to unbind from your current selection.
Do note that if you are currently bound to Facebook, this will be counted as one of the limited binding attempts. Once you've unbound your game data from Facebook, you can no longer use Facebook as a binding method if there are no remaining binding attempts.
Do note that if you are currently bound to Facebook, this will be counted as one of the limited binding attempts. Once you've unbound your game data from Facebook, you can no longer use Facebook as a binding method if there are no remaining binding attempts.