It's important to know the difference between what a WDH will do to the car versus what upgrading the suspension will do to the car. Apologies to those that know this already.
Normally the car has a certain amount of weight on each axle. Nissan's manual even defines this in its specifications sheet. For example, a 2014 STX 550 diesel can have up to 1530Kg on the front axle and 1717kg on the rear. Yes, the combination exceeds the GVM (in this case 3010Kg) but you are able to carry 906Kg in the tub (this model) which obviously places a larger load on the rear.
The problem is that when a load is placed behind the rear axle (caravan/trailer) it's not just a load on the rear - it's a lever trying to lift the front axle up. A fifth wheeler, for instance, does NOT cause this problem so there's no need to account for it. As an aside, in a fifth wheeler's case, all you need to do is upgrade the rear suspension and the car's ready to go.
Because the weight is actually levering the front wheels up, changing the rear suspension isn't going to fix it - the lever is still there regardless of how hard or soft the springs are. In fact, with soft springs - or suspension that hits or sits on the bump stops with the trailer attached - the effect is far worse and more dangerous as the lever is now acting on a solid rear since there's no suspension travel to allow some flexibility.
Enter the WDH. It's a misnomer - it doesn't really redistribute the weight, but it does redistribute the effect of the weight. The spring bars twist the hitch upwards around the rear axle, effectively pushing the front wheels down. Setting one up properly is vital. Hayman Reese had a video of it and I've posted a step-by-step somewhere here too. Basically you get the ball to the correct height (especially for double-axle vans, the chassis should be level or very slightly nose-down) then adjust the bars so that the front and rear suspension settles within a few mm of each other once the bars are hooked up.
It's my opinion that towing a heavy van without a WDH is dangerous because of the reduced steerability and braking. It's especially noticeable when descending dirt/gravel hills.
Sorry if I seem like I'm harping on this, but it's important for everyone's safety!