Primary purpose of WDH is to restore weight lost from front axle of the tow vehicle. Knowing the GVW of the trailer, gives them a good idea of what the max tongue weight could be. Restoring front axle weight is WDH's biggest impact on sway control.
When the tow vehicle has too much weight removed from the front axle, it losses stability and steering control. Something simple, as your breathing causing your hand to subconsciously move, ever so slightly, could cause the front end to wiggle. That would cause the rear end to wiggle and, in turn causes trailer sway. That's where the TW and Bar strength comes in.
Additional sway control is more based on length, than it is on weight.