You can't get all the boost you want from a supercharger... you can only spin it so fast. Turbo on the other hand, you can get huge power from but you have to get it spooled. So, if you have both the supercharger boosts when the turbo can't and the turbo provides the big boost the supercharger can't deliver.

There are 600 WHP MINIs that use a twin charged setup.