Not the only or maybe even the best explanation, but I'll take a stab at it.
Having that information would be useful/necessary when investigating why differential services are already being provided. As much as we want the gov't (at all levels) to be color blind, it isn't. I live in the south and racism is still evident, and I expect always will be to some degree. When a voting district has been gerrymandered in such a way as to weaken the impact of african american voters that census information would be useful to use as a tool when outing the people who made the district changes. Given racial sensus information an automated system could even look for patterns that show where previous or current generations have screwed with boundaries for similar reasons.
It's a lot harder to detect such manipulations without that data unless you live in/near the affected area.