Support weapons normally cannot be used from the hip - they must be fired from a tripod, or by prone characters using a bipod. A weapon harness is a strap-on carrying arm that suspends the weapon and compensates for recoil, making it possible for the user to shoot such a big gun from the hip.
Unless the user has a TargetScope, a weapon worn in a weapon harness can never have a range better than normal (20 meters).
Weapon harnesses are unusual except in the military, and cost an item schtick to have.