What is a supply pressure gauge used for?

A To show how much pressure is in the air tanks.

explanation

All vehicles with air brakes have a pressure gauge connected to the air tank. If the vehicle has a dual air brake system, there will be a gauge for each half of the system. (Or a single gauge with two needles.) Dual systems will be discussed later. These gauges tell you how much pressure is in the air tanks. [CDL Handbook: The Parts of an Air Brake System]