Mikes said it all I think. My turbo conversion comes with one as standard because they are important. Lets say youve got the turbo spinning on full boost at 100,000 rpm which isnt that fast for a turbo. You let off the accelerator to change gear, the throttle plate closes, suddenly all that boost between the turbo compressor and the throttle body, cant go anywhere except back through the rapidly spinning compressor. To avoid backing up the compressor and obviously straining it by huge sudden reversals of forces on it, the dump valve opens whenever there is boost in the circuit and the throttle closes, letting out unused boost pressure and letting the compressor wheel carry on spinning as before.