The PURPOSE was to make a ten round rifle into a single shot.
The REASON was because officers know best. Or at least they thought they did, and without orders from one you were not allowed to fire more than 'one at a time', you know those 'uneducated' schmucks can't be expected to ration their gear or use their brains.
Back in the day ammo was a lot more 'expensive' than today. But in all truth, this concept is pretty much still standard in some backwards nations. Our C7's have full auto, but the Americans still insist on a lot of their M-16's having 'burst' fire... three rounds max at one time. Cuz again, servicemen are wasteful and apparently incompetant. Same ####, different pile.
Officers know best.