Simples. Everyone is on board and you want to go early but you cant cos your handling agent has dumped the loadsheet in the cockpit and fled off to the next aircraft and they won't answer on the VHF frequency to bring forward your TOBT which in turn generates you a new TSAT which is "hopefully" closer to what the actual time is now when you want to go. OR you are running late and miss your TOBT and TSAT and again no one answers the VHF so you sit there and go out of hours.
Its a great system. Must have been designed by Germans.