Hinline is correct.....first and formost it quiets the muzzle gasses, regardless of the speed of the bullet.
there are 3 noise factors: 1 mechanical noise of the firearm, 2 the muzzle gasses, 3 the bullet flight noise.
of all of these the muzzle gasses are the loudest, and the main thing a suppressor is made to reduce(although some designs encorporate ported barrels to reduce the speed of the bullet, to control bullet noise as well.
the factory MP5SD is one example, but MP5's with "muzzle cans" do nothing for bullet velocity reduction.
All the special ops guys you see with cans on thier M4, are using regular supersonic ball ammo in them, but it protects thier hearing still...and bullet flight noise is a really odd thing in combat...its a vantriloquist act in action lol it never sounds to the target to be comming from the actual direction that it is( often making them shoot in all directions, and run at you thinking they are breaking contact( they are many stories of this from Viet Nam with the old Sionic suppressors used for night ambushes on M16A1's and M14's equipped with starlight scopes.
a suppressor serves the same exact role/job as the muffler on your car