Cartridge alignment. Gas sealing. Recoil reducing. Safety (an extra layer of steel surrounding the rim of the cartridge). The easy answer.......gimmick.
Do the disk do all of these things? Probably not. Back in the day it was a selling point and my best guess is....they made them like that, so lets continue to make them like that. Lots of gunmakers were guilty of other gimmicky things too. Darne was no different.