Originally Posted by David Cairns

A supercharger just adds a fixed amount of boost to the existing atmospheric pressure. At sea level, my boost gauge reads 7 lbs but at 10,000 ft, it is down to 5.5 lbs. Turbos (and perhaps some centrifugal superchargers) are different, they frequently vent extra boost with a waste gate. This is due to peaky boost curve that they produce. By allowing the boost to come on sooner and venting off the extra boost on the top end, they can improve the boost curve. It also helps with higher altitudes as the waste gate allows boost up to the same internal PSI before venting.