What does a regulator do and why does it matter?

A regulator sits between the high-pressure reservoir and the firing valve.

It bleeds reservoir air down to a fixed lower pressure (the setpoint, often 100 to 200 bar) into a small chamber called the plenum, so each shot fires from the same pressure regardless of how full the reservoir is. The result is a flat shot string with very low extreme spread, often under 10 fps over many shots, until the reservoir drops below the setpoint. Most modern serious PCPs are regulated.