when your throttle plate closes, the blow off valve opens so the pressurized air has someplace to go. if the air stayed in the intake tube it would surge back into the compressor housing and possibly take off a few fins.
you can fit a blow off valve to a supercharger, but they usually have a return line that puts the pressure back before the compressor so it holds some pressure - since they are positive displacement pumps and are pretty much regulated by engine rpm they will pump less air at idle (obviously)
you can have a Supercharger with a BOV. its not unheard of
a supercharger is more reliable, less heat. get more HP from a turbo
a turbo is more heat, not as reliable
Im a Xbox 360 fanboy...and damn proud of it!!