You'll find that a typical class IV or class V trailer hitch will have different ratings depending on the use or non-use of a weight-distributing hitch.
It doesn't affect the base vehicle's tow ratings, but you're not allowed to exceed the capacity of the hitch, either, even if you're within the limits of the truck.
I realized something about weight-distributing hitches which may not be clear to everyone.
Here on TBN we often talk about using
ballast on the 3ph not only to permit the loader to actually pick up its weighted load, but also to reduce the load on the front axle.
A trailer's tongue weight acts similarly to the weight in a loader, as it's a weight pushing down beyond the axle (behind the rear axle in the truck, as opposed to in front of the front axle of the tractor), and as a result, it imposes a lever action on the truck with the rear axle serving as a fulcrum - this is why the front of the truck comes up, with some of the front-end weight of the truck getting carried on the rear axle instead.
Using the weight-distributing hitch, you're in effect forcing a rotation around the trailer hitch (actually where it's bolted to the frame of the truck, which isn't exactly a pivot point, and in this case the pivot ends up actually being the front axle of the tow vehicle) in the other direction by pulling up on the WDH bars, which "moves" the weight forwards on the truck - putting more of the tow vehicle + tongue weight onto the front axle instead of it all behind on the rear axle.
Here's a ridiculous situation that's possible with a WDH where they actually put the entire tongue weight
plus more! on the front axle of the towing vehicle:
(front wheel drive car can literally pull this trailer without dragging the rear wheels - though it's not going to be able to turn very well.) As you can see here, they've actually put so much tension though that system that they've put the tongue weight plus all of the weight that's typically on the rear axle onto the front axle!
A heavy trailer could potentially overload the GWR of the rear axle of a truck when the entire tow rating hasn't been satisfied yet (especially if that trailer is loaded with an inflexible load that forces you to have a very heavy tongue... like a pickup truck on a short flatbed trailer perhaps). Using a WDH allows you to put some of that excess tongue weight onto the front axle so that the heavy tongue doesn't overload the rear axle.
The weight distribution hitch also puts weight on the trailer axles. The longer the spring bar, the more weight gets transferred to the trailer.