How would they know if you clock in, work for a while, then leave and come back later to clock out?

And on a related note, when there's nothing to do and you know there won't be anything coming your way, why is it acceptable to sit there on your ass doing fuck all while pretending to look busy, but not okay to leave the premises (while still being available on the off chance that anything comes up) and actually do something enjoyable with your time? Either way, no work is getting done so why does it matter where you are.