It was a systems test. To clarify: They wanted to make sure the brakes could work under worst-possible-conditions, which included not using the engines for assistance, and no immediate ground response.
GarlicMyfi
: It's a high-tech projection technology, where they do precise mapping of the target surface, and create the animation to take advantage of that surface. This is where I first saw the effect:
This is, however, the first time I've seen an example with a moving target! They start the projection while the plane is still being towed, not after it stops. That's some amazing timing there.
I missed the line about WHY they couldn't engage reverse thrust (at any point).