Why do you have to use premium gas in your supercharged 2001 buick ultra?

Yes. The computer is set up for Premium gas. The car won't run as well on regular. Additionally a supercharged engine by nature forces more air and fuel into the cylinder per cycle creating greater compression of the fuel/air mixture. This creates greater heat. This greater heat can cause the air/gasoline mixture to ignite prior to the spark plug firing. Higher octane gas is not as combustible as lower octane gas. It resists premature combustion better and thus your engine fires when it is timed to rather than at some random time when the fuel air mixture gets hot enough to explode.