Going back to the SeaHunt era BC’s didn’t have power inflators, my old Conshelf didn’t even have an extra low presure port to add one. To add air you took the reg out of your mouth, unscrewed the BC inflator hose, many used lifejacket valves, added air, closed valve and put reg back in mouth usually getting a mouthful of saltwater. To get the saltwater out of the breathing side (the side Jeff never cut) of the double hose reg you did a slow barrel roll to move the water to the exhaust port.
After doing that a few times some Aussie had a better idea and came up with the single hose reg but the down side was the bubbles in the face.
Some Swede came up with the side port to deal with the bubbles. My Oceanic Omega keeps the bubbles out of my face but breathes wet when looking to the left, every design has a downside :(