There are two types of regulators. High Pressure Regulators(HPR) and Low Pressure Regulators(LPR). The HPR is used to reduce the pressure coming from the tank. Say the tank puts out 600 PSI, the HPR will reduce it to 250 psi. Then the air flows to the LPR, which is reduced from 250 PSI to about 120psi.
Consistency is the important thing I believe. If the regulator isn't consistent, it will not be as accurate.
So, yes, they do adjust, stabilize and control the velocity, but there may be easier ways to adjust your velocity. Again, I would have to know what type of paintball marker(gun) you are using before I can really help all that much.