that pressure switch is also one of the parameters for the air conditioner, along with
the wide open throttle cutout.
if that detects too much pressure the ecu will turn off the a/c compressor momentarily.
this will reduce load on the engine so that it does not stall,, i.e. parking lots.
at least thats what the service manual says.
sure would be alot nicer if that switch was actually a nanny that tells you if you run out of fluid so that you do not fry your power steering system.