For your uses (or mine), you are right, they serve no purpose. Their design, with the serrations and venting, is for breaching - place the serrations against a door or other item that you'd like to "open", load a breaching round, pull trigger, instant lock pick. :twisted:
Where you can get one? Look for a thread called "Hammered 870" - I recall there being discussion there about a few different makes of muzzle breaks.
Edit: To clarify the above, I am trying to indicate that they DO serve a purpose, just not for the average Joe, who would have a hard time finding breaching rounds, and an even harder time finding a legit use for them. I'm well aware of the true use, just wanted to make sure I didn't come off sounding like I was uninformed.