Nothing, they are just drawing and shading quads as a practical GPU benchmark. Each pixel could be shaded with a pseudorandom color generated as the hash of its screen coordinates.
> In many graphics applications, a deterministic random hash provides the best source of random numbers.
So, why do people need cheap random numbers in graphics? Is that to create some sort of fuzz/texture/visual effect? Or is it to implement stochastic animations? (I know nothing about graphics)