Basically, it allows the pressure that has built up when on throttle, to be released. If the pressure wasn't released then it would stall the turbo, and break it quite badly. A diverter valve releases the pressure back into the intake system, where as a blow off/dump valve let's the pressure back out into the atmosphere, hence you get that annoying noise that Skylines and Imprezas make.
An improved Diverter valve will not leak and let boost pressure out of the system, making the engine more responsive and build boost quicker. It will also be better for a remapped engine as the standard ones are quite fragile (depending on year of make) when asked to hold more boost pressure.
