You car probably gets some extra power by using an engine with a higher compression ratio. In some cases of engines with high compression ratios the computer can compensate by delaying the spark timing if the fuel grade is lower than premium. You lose a little power is all. However if the owner’s manual says “premium required” then the car’s computer is not able to compensate for lower fuel grades, and you run the risk of engine pinging (pre-detonation) and that can do major damage to the engine’s internal parts.