Nice post, and probably an accurate conjecture as to why used.
One winter a bunch of us in Rogers Rangers, Boat 17, went out on a snowshoe trek and, somewhere on Black Mountain, NY, and found a dead tree about six feet tall. One of the guys got us to do a hasty stake shoot just to see what some elements of battle in the snow might have been like. Haven't a clue as to why I had a bag like this, made of plain old cowhide and poorly dyed. But darned if that bag did not make one world of difference when throwing everything in the muzzle and coping with the 25 below , sunny weather, speed shoot. I hadn't yet learned the virtues of a simple cartridge containing both ball and powder, so I had paper loads on one side and ball on the other in the divided bag.
Since you don't turn a bag like this inside out, it's a simple stitch that works great. If made of cow, all you need to do once stitched up is wet it and bulge it with your fingers. If you elect not to dye it, some neatsfoot oil and a lot of sun will give it a great patina to start off a lot of trail memories as it ages.
Thanks, T.C.