When you enable mac pinning on the port, mac authenticated clients are "pinned" in the switch authentication table for that port and will maintain their authentication until either the port flaps/bounces, or the switch reboots.
Since some devices like printers or medical devices go into a "sleep" mode after a period of time or do not send out any kind of "keep alive" packets, this will allow those device to stay authenticated when "awoken". Basically the logoff period is disable on the port for that specific client.