It stores more gas in the front block before it goes into your valve. They control the volume going into the valve by means of screwing and unscrewing. BTW, they're called Low Pressure Chambers (LPC's).
if the gun is gassed up, the chamber will shoot into your eye and out the back of your head. if it isnt, you will have a spring come out, followed by the cupseal which is connected to the spring. (also depends what gun you got).
There is no need for one, though later Cockers have a larger internal valve chamber then older versions.
LPC are mostly just important on ram-valves like Timmys/Imps/BM2Ks because the ram and valve need air at the same time, and there is a possibility of the system being starved (ie causing bolt-stick, and drop-off).
100% useless on HP guns, but it makes a nice marketing gimmick since it costs nothing to add to a gun.