Hint: they won't lift your load off your legs, but they ought to lift the load off (the top of) your shoulders.
They are an adjunct to your hip belt, which ought to be transferring all your pack weight to your hips. Ideally, your shoulder straps are only stabilizing your pack from falling backwards, and they will only press against the front of your torso and shoulders, not heavily down on your shoulders.
Necessary? It all depends!