When MacGyver needs a tool and doesn't have one, he just throws some random parts together and the magic of television sorts it all out. Ford had a similar problem, but since the company has firm standing in reality, it had to find a stronger solution, and the result is itstowing dynamometer.
If you need to test your towing prowess on various grades, but only have flat land available, that's where this kind of dynamometer (dyno, in automotive parlance) comes in. It simulates towing on a grade by using electrical coils to vary its resistance. To keep it light, it's able to imitate the effects of gravity.
As you might note, this is a bit different from what most people think of when they hear a dyno. That's a "rolling road" type of setup, which measures a drive axle's tractive effort. From there, it calculates the car's torque and, by relation, horsepower. This...is a bit different.
Ford picked up this specific rig because its other dynos were incapable of providing the correct resistances to test its new line of Super Duty trucks, or so Ford says. The company could have built this dyno just because it would be a neat thing to do. Couldn't blame 'em if that were the case.
So why not just head to Davis Dam or Townes Pass, the two very unique locations Ford's trying to emulate? It's all about time and money. Using this dyno at Ford's testing ground in Arizona is both cheaper and more efficient than closing roads and shipping heavy-duty trucks (plus personnel, plus testing equipment) to random locations.
Editors' Note, 1 April 2016, 12:14 p.m.: This article has been amended to reflect the fact that Ford purchased this dyno from an independent manufacturer. Ford itself did not build the dyno in this story.