Some subsonics are quieter than others out of rifles, at least with the can on.
With a pistol, nearly everything is subsonic and pretty quiet. I run or ran it on both, bolt action guns are much quieter than autos where rifles are concerned.
As to why? There is no comparison between suppressed and merely subsonic. With a good bolt gun, about all you will hear is the firing pin fall. I can think of several instances that could be desireable but they may not be for everyone.
