I think the spacing is part of it, but as the previous post pointed out the same thing can be done with washers or O rings. Hanger bars are machined bars that bolts on where the forearm used to bolt. I leave them on the barrels once installed. The forearm bolts to the hanger bar. This way the torque on the barrel is constant when the barrel is removed from the frame and then re-installed.
I am a Contender shooter and haven't had any trouble with poi changing when removing and reinstalling barrels. There have been several posts here discussing problems with Encores in this respect. I suspect hanger bars at the least remove a possibility for variability.
