Air compressors have two gauges because you need to know the pressure inside the tank and the one that comes into your tool.
One of the gauges is the tank gauge, which will always tell you how much PSI is swirling around in the tank.
This gauge will show a reading based on the maximum capacity of the compressor. If you have a compressor, you mostly use around the house, it likely has a gauge with 200 PSI max.
As you turn on the compressor, the indicator on this gauge starts to rise (it should always be at 0 when the compressor is turned off).
Compressors have a factory-set cut-out point (which varies from 90 to 175 PSI). Once your compressor fills with air and reaches that point, it will no longer compress.
The second one is the regulator gauge, which should be located near the compressor’s regulator knob. As opposed to the tank gauge, this one will indicate the PSI coming out of the tank and going into your air tool.
This gauge is highly important because different air tools have different PSI requirements. You don’t want to use too much pressure with your tools because it wears them out faster.
Because of that, you can adjust the regulator knob to determine how much pressure you want the air compressor to release.