Its a constant fuel pressure reg, so it will hold a specific pressure (can't remember what it is) or at least try and hold a specific pressure, with atmospheric pressure as its input .

Showing it vaccum/boost helps in two ways,

One it means it sees a vacuum off boost, and the held fuel pressure is lower, and Two, it sees a positive pressure on boost so holds a higher pressure.... enriching the mixture.

This all means that the fuel pressure set in the electronic fuel pressure reg can be set lower... meaning your car should have the correct on boost fuelling with out the MFI's failing feature of being pig rich off boost.

It looks at atmospheric pressure because the higher you go the less oxygen there is meaning the less fuel you will need, its there so the car can fuel correctly in all situations if you get what i mean????

The con is that the fuel pressure reg is not calibrated to see vacuum and boost and so the fuelling still isn't as accurate as it could be (if the reg was designed this way).

Rob,