Why is Pri Hyd buttons labeled as "ON/FAULT" and not just ON/OFF?
That should be fairly clear from the sim..the "fault" bit on the switch is a
light not a label. Take a look when you are in the box..turn the primary pumps on and the fault
lights will go out when appropriate, but it/they will remain on if there is a system fault...so it is not just an ON/OFF switch..., it's an ON/off switch with an attached FAULT light.
As for the Bleed switches, it's a bit more vague..ARMing the the APU valve puts it into the hands of system logic, whereas turning the engine Bleed valves ON will open the bleed valve as long as bleed air is available and there are no faults..but TBH I don't think there's much practical difference between the two.