A waste gate actuator is a vacuum operated device that opens a valve inside the turbo to let exhaust gases out of the turbo whenever the set boost level is reached. When your waste gate actuator is designed to open at 14 PSI, and once the engine is pressurized at 14 PSI your waste gate actuator will open and then let all of the exhaust gases exit the turbo without spinning the compressor fins as the boost level that its designed for is reached.
When should you use a aftermarket waste gate actuator ? When you are at higher boost (ie, 16 + PSI) levels you'll notice that your boost level will peak at a certain PSI but then fall between 5000-6000 RPMs. The best way to create a steady boost level or make it so that it wont fall off as much is to get a stronger waste gate actuator. This will allow you to have a higher more steady boost level between 5-6k RPMs which is where power is critical when your racing.