Pilot unloader valves are necessary for the proper functioning of the continuous run type of compressors.
Some compressors are not able to stop and start frequently enough due to mechanical limitations, such as might be demanded by a high volume compressed air use plant. If the cycle time between high and low air pressure set points is too frequent, this is negative to that type of compressor, increasing wear and short life expectancy of some of the parts.
If your plant has a demand for compressed air that means that your compressor cycles on and off more than 15-20 times per hour, you have reached the frequency-benchmark for considering moving from a Stop & Start type of air compressor to continuous run style.
However, when the compressor runs continuously, it would continue to compress air into its receiver well past its pressure-safety zone, with potential catastrophic results.
This is where the Pilot Unloader Valve comes into play.
The benefit of this is reduce wear, less heat generated, a reduction in energy costs, and also stops the compressor from continuing to compress air into the receiver or plant air mains.
When the air system lower 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.
The Stop & Start type air compressors have unloader valves too, but of a different sort. Information on them is here.