Some compressors are not able to stop and start frequently enough to supply compressed air to high volume demand plant. If the cycle time between high and low air pressure set points is too frequent, this increases compressor wear and can reduce life expectancy of some of the compressor parts.
If your air compressor cycles on and off more than 15-20 times per hour, you have reached the frequency-benchmark for that type of compressor. It's time to consider moving from a Stop & Start type of air compressor to continuous run style.
This is where the pilot unloader valve comes into play.
With the compressor running with no load, there is reduced wear, less heat generated, a reduction in energy costs.
The pilot unloader valve opening also stops the compressor from continuing to drive air into the compressor tank or directly into the plant air mains.
When the system low pressure set point is reached, the pilot unloader valve closes, and air is once again being compressed into the receiver and the air mains until, once again, the high pressure set point is reached and the pilot unloader opens.
All the while, the compressor motor continues to run, eliminating the too-frequent compressor start/stop cycle found in high-air demand shops.
The Stop & Start type air compressors have unloader valves too, but of a different sort. Information on them can be found here.