[rheavn]

If you are going to work on the air system, you can shut off the valve and your seal will stay inflated even when you drain the air pressure.  It also can be used to check for air leaks beyond the valve as you can use the regulator gauge as a leak down tool.  If they were replacing your seal they could do so without draining all the air from the coach.  Maybe a few more that I haven't thought of........................