Here in the South at least, they want that info so they can draw districts such that they have people of all one race in them, insofar as possible. They say that down here, people are represented better by congresscritters that look like them, but that doesn't apply in the more enlightened North, so they get square districts, or broken down by party, like would make sense.

Thus saith my gov't professor, though perhaps not quite as sarcastically as I have related here.