They make guns quieter and reduce recoil. All around more pleasant to shoot and reduce hearing damage. Guns are loud enough that exposure to even a few rounds in close quarters can cause hearing damage so suppressors should be in regular use as safety devices. They don't make guns safe without earpro, they don't make them silent, they don't even make them that quiet. You will still know if one goes off nearby. This is one of the things about hollywood that pisses off everyone who knows squat about guns, and why we try to refer to them as suppressors rather than "silencers".