There are a few reasons to use a flash suppressor. One reason is to protect the shooter’s night vision. When a rifle is fired, the muzzle blast can momentarily flare up and shine a bright light in the direction of the shooter. This can temporarily blind the shooter and make it difficult to see what he or she is shooting at. A flash suppressor helps to reduce this effect by channeling the muzzle blast away from the shooter’s line of sight.
Another reason to use a flash suppressor is to help conceal the shooter’s location. When a rifle is fired, it emits an unmistakable muzzle flash that can give away the shooter’s position. A flash suppressor helps to reduce this effect by dissipating the flash so that it is less visible to the naked eye.
Finally, some shooters simply prefer the look of a rifle with a flash suppressor. Many consider it to be more aesthetically pleasing than a plain muzzle.