They will tell you that its supposed to help lower your intake temps. And while I dont personally feel it does, it does lower the temperature of the Throttle Body since it is isolated from the blower. The argument could be made that that helps the sensors mounted on the TB, but I think they are durable enough to deal with it.
The other purpose for it is that it gives you a good place to install nozzles for water or nitrous injection. Other than those it doesnt seem like it has much else in the way of purpose.
Furthermore, it adds another gasket to the set up and another place for potential leaks. I will also add that the studs in the supercharger are M6 x 1.0mm studs that are mounted into an aluminum case. That spacer seems to take a bit more pressure/torque down to seal properly and that runs the risk of stripping those things out.
