A regulator collects and feeds a measured amount of compressed air to power the pellet, but because it is also capable of keeping the pressure of that air constant - whether tapped from a fully charged reservoir or a near empty one - the shot-to-shot consistency is far higher. Regulating the air supply also regulates the level of force imparted to each pellet and this keeps the muzzle energy behind each shot as near as possible to identical throughout the effective part of a reservoir's charge of air.
This was taken out of this months Airgun World.