You are correct that you cannot completly supress a supersonic round. They do greatly reduce the bang of a fired round but the sonic boom still exists. If subsonic ammo is used it is very quiet but you do lose some of the effectivness of the rounds fired. Suppresors effectivly allow the pressurized gas behind the bullet expand and cool before exiting the end of the gun. This eliminates the bang or crack heard when the gas is released when the bullet exits the barrel. They do also eliminate muzzle flash and greatly reduce recoil. They are used in a military role to help reduce hearing loss in enclosed areas, to help with stealth, to help identify friendly fire from enemy fire, helps with useing night vision by eliminating flash, and a host of other reasons.(I am no expert). IMHO supressors in general are a fantastic tool for all shooters and one of the greatest developments ever devised for firearms. Now if we could only get rid of all the NFA nonsense.