"Moments are usually defined with respect to a fixed reference point; they deal with physical quantities as measured at some distance from that reference point. For example, the moment of force acting on an object, often called  https://en.wikipedia.org/wiki/Torque - torque , is the product of the force and the distance to the object from the reference point."

So its not enough to just say that the torque being applied to the TV via the wdh could be applied anywhere on the TV.  You have to pick a point you're applying the moment around. In the case of the wdh that is the ball, the rig pivots around that.  

Second, just like the ground is creating an opposing force countering the weight of the rig so it doesn't fall to the center of the earth, there also has to be an opposing moment keeping the TV from continuing to rotate into the ground counterclockwise (as seen from the left side) when the wdh is tensioned. In effect that acts to increase the down force on the TV front axle and reduce the down force on the rear axle. 

Now on the the trailer. Since the spring  bars are being tensioned between the hitch attached to the TV and the trailer A frame, there is a clockwise moment being applied to the trailer and that is equal and opposite to the counterclockwise moment on the TV. That moment, also applied at the ball, is stopped by the ground pushing up on the trailer wheels, effectively increasing the load on the trailer axle, 

So, the wdh does redistribute down forces from the TV rear axle to the front axle and the trailer axle. The proportion getting redistributed where depends on the distances from the ball to each of the three axles. The total getting redistributed depends on the tension on the wdh bars.

So while the video dude is correct that the weight on the ball doesn't change, the down force on the rear axle does indeed go down with a wdh. With stiff enough wdh bars and frames that can take the load, you can take all the weight off the TV rear axle and suspend it in the air. There is an old wdh ad photo somewhere of an Olds Toronado with its rear wheels removed towing a trailer, the back of the Olds is just hanging there.

What's important here is the point of the wdh is NOT to just level the TV. It is NOT doing the same thing at all as adding helper springs to the TV rear axle. Adding helper springs does not increase the load on the front axle or reduce it on the rear axle. Driving around with too little weight on the TV front axle is dangerous. 

Whether you need a wdh with your particular rig or not will depend on the specs on your TV and how your trailer is loaded, If the TV is sufficiently heavy and/or has a long enough wheelbase (remember that the moment is the product of force x distance so a long wheelbase means that the load on the front of the TV won't be reduced as much to counter the tongue weight) then the wdh will be unnecessary.

If you do benefit from one that does not mean your TV is marginal, it just means that the wdh is part of the design requirements for that vehicle to carry that load. The point is to be sure to understand the specs and be comfortable with the way you have loaded and set up your rig. 

And no, you're not going to  break your TV  going through a swale by using a wdh within the vehicle manufacturer's specs and limitations.   

I'll include the link to this calculator again which I found very helpful in understanding what's going on and setting up my rig the way I wanted it. 

https://www.ajdesigner.com/apptrailertow/weightdistributionhitch.php - https://www.ajdesigner.com/apptrailertow/weightdistributionhitch.php