This part serves two functions in your Vortech or Paxton supercharger. First, it provides the initial spacing necessary to space your impeller up near the volute to make power (you may add additional shims above this washer); and second, it is the sealing surface for the mechanical face seal to seal against.
If you have replaced your seals and still have a leak under the impeller, have a look at your thrust washer. If the surface of it is grooved and worn, that could explain why the seal cannot seal against it. Our part is especially hardened so that it can withstand the wear of the seal against it.