The same reason people do box squats, speed work, use reverse bands, and about a billion other tools in the toolbox. To build reactive strength. This is basically the ability to overcome the reactive forces of your body when performing a physical task. Like slowing the bar down to quickly reverse it into the concentric phase during a squat or a bench. If you are not in control of the reactive forces during eccentric/amortization phases then you get buried under whatever weight is in your hands/on your back.
The reactive force that needs to be overcome in the above video is force generated when my feet hit the ground right before the jump onto the 20inch tire. Thats a 75lb bar on a 280lb body. The reactive force of the ground when my feet hit is probably about double that weight. Overcoming that force builds reactive strength.
This is the first time I have tried this exercise so the transition from landing to jumping is a little slower than I would like it to be.