Collected my fair share over the years from the runway taxiways and aprons, always been taught that a valve cap = Dust cap if valve is exposed brake dust grime can build up, upon inflation this may contaminate the valve core and stop it seating properly causing it to leak.
The other valve caps I know of are the ones held on to a valve spring by collets which secure the valves in cylinder heads if they let go you are in a spot of bother
