When a caravan is loaded correctly and agreed, at the correct nose weight, that figure is at it's optimum when used at the correct height ie. The one that the towing vehicle tow bar height is set at.
As made clear (04/05) by John L's response to "Noseweight Question" in the 'Technical Section', due to the fulcrum effect, if the height of the caravan hitch is raised, the weight transposed through it is reduced and in a ridiculously extreme example, the rear of the caravan will drop to the floor with a negative weight value being put on the tow bar ie ;- Having the effect of lifting the rear wheels of the tow car.
If you weigh the noseweight of your van at it's normal towing height on say, a set of bathroom scales with a piece of wood cut to length to mirror your tow ball height, then remove the wood and weigh the nose weight at ground level direct on to the scales, the latter will provide a greater figure.
Therefore, if the tow bar is lowered to below it's normal towing height, the weight transposed through it will increase, as just described, and when the rear of the towing vehicle sags, the weight transposed through the hitch will increase.
The effect of any weight pushing down on the tow bar, has an effect on the towing vehicle, and again using a ridiculously extreme example, if you pushed down with enough force, the car would pivot around the rear axle and the front wheels of the tow car would lift clear of the ground.
Self levelling suspension, retains the optimum height of the hitch and therefore prevents the 'increase' in weight transfer as the hitch drops due to the effects of loading.
This allows the front wheels to remain in full contact with the road and therefore has an actual effect of transferring some of the load from the rear axle, back to the front axle.
With reference to the rather more complicated air suspension set ups, these also allow the transference of loads, not only from front to rear, but also from side to side.
When a vehicle corners, some of it's weight and the 'G' effects of cornering transfer to one side of the car or the other, depending on which direction you are cornering.
This is what gives body roll, however due to the suspension, the car body is allowed to tilt while the four wheels remain in contact with the road surface, allowing the car, under normal driving conditions to corner under control.
In extreme cornering, the effect of the body roll can be sufficient to lift the inside wheels from the road surface, reducing tyre grip and causing tyre squeal or eventually a skid or rollover if enough tyre contact is not maintained with the road surface or if the body leans too far.
The stiffer suspension seen on sports cars, along with other measures, reduces body roll and helps keep the car on an even plane when cornering at higher speeds.
The air suspension sensors on my last VW Toaureg automatically detected the amount of body roll and stiffened the suspension as required to help keep the car on a level plain, thereby transferring some of the sprung weight back to the opposite side of the car, reducing cornering effects and keeping the outer wheels in contact with the road surface and retaining that 'all essential' grip.
So in essence, self levelling is a benefit to driving and towing, including both handling and braking aspects, as a tyre that is losing traction with the road surface has a reduced braking efficiency and is less likely to keep the car on the road during cornering, however, self levelling will not allow you to exceed the recommended nose weights of your car or caravan.
PS. I do not usually drive or tow to the extremes cited above, they are purely examples.