"What practice application will this serve in a desktop that a video card cannot do?"
GPUs have much smaller "instruction sets" if you will, and cannot trace out solutions through iteration, or have large amounts of core specific local memory (state variables, flags, etc). With GPUs you tend to know the operation they will do start-to-finish ahead of time (aka there is no conditional branching).
Granted, there are ways/interfaces to do CPU workloads with GPUs, albeit less efficiently (work per watt probably an important consideration with these cards).