I think that to the extent they "anchor" your pack to your waist, they can help take some weight off the shoulders. But the effect won't match even 1-inch belts with fabric hip belt "wings".
My old GoLite Dawn pack and my current Z Pack Z1 both came with one-inch webbing hip belts — to which I added silnylon pockets and have them do double duty as "wings". My shoulders feel comfy when I use these packs on multi-day hikes with loads at 20-25 lbs.
My rough guess for 3/4" belt without any wings would be a comfort load of around 15 lbs. — maybe a little bit more. Just a guess.
Finally, as a comparison, using a pack without any belt, my shoulders start to whine when the load gets much over 10-12 lbs.