Subsonic means having a velocity less than that of sound (the opposite of supersonic). .45 ACP is inherently subsonic but making the 9mm Parabellum, or .40 S&W for that matter, (an inherently supersonic round which depends on higher velocity for its effectiveness) slower is stupid and real world shootings have illustrated this. The only benefit would be for suppressors, as mentioned, and to avoid the extra noise component of the sonic crack when the bullet breaks the sound barrier (though muzzle blast is all the same to your eardrums). Simply, if you want subsonic, just get a .45; it doesn't require speed to make a big hole every time.