I've been wondering the same for some time now as well! It's near the old incline and not far from one of the adits (now blocked) but there's no wear marks to indicate cables moving through it and no significant infrastructure around it to give any clues.
Incidentally we were in Penarth yesterday, and if anyone needed those large white spray painted arrows and "WO" signs to navigate the mine you'll be out of luck; amazing what a couple of sturdy brushes and mud can achieve...