the upper reciever usually holds a recoil spring and bolt carrier. in this case, it's just to hide unsightly pieces such as the gas system, parts of the barrel, etc. the belt-fed is simply an open-bolt version of a standard under-barrel grenade launcher. The only immediately obvious advantage would be the ability of semi-automatic fire. The idea is that a hammer assembly would set off the grenade and then a battery-powered gear inside the RIS would advance the belt. It's not supposed to be realistic, belt-fed implies fully automatic, which would be a hilarious idea for an under-barrel GL.