[Jared Russell]

When the cylinder is extended, the volume that the pressurized air in your pneumatic system must fill increases (because pressurized air needs to fill the inside of the cylinder behind the plunger). Once the entire system (on the working side of the regulator) reaches the desired pressure level, there is no need to keep the compressor running except to replenish any air lost due to leaks.

This goes for both double-acting and spring-return cylinders - once you are extended, all you need to do is maintain pressure.