Its third panel folds between the weapon and the storage pouches and ammo loopholes, so it serves the dual purpose of protecting your rifle's finish from the stuff that you put inside the case and providing a wide work area when you're laying on the bag. It also works well to permit the carriage of two weapons in the bag without them rattling against each other.
Heavy duty compression straps cause the pack to ride quiet and tight, and it can be compressed completely flat. In this photo, they are routed through the scabbard compression loops, which allows you to selectively compress the scabbard. Also note the side handles; these make it easy to get the loaded pack out of a vehicle, and for mounting the pack with a heavy load.