Someone may want to know if you are a Democrat or Republican (for advertising, for gerrymandering, whatever). That person may not be able to ask you a direct question like that though (or may feel that you may lie about the answer to such a question anyway).
As such, they likely carry out occasional surveys asking people who are Democrat or Republicans for their opinions on something else. Once they find something else that can strongly correlate your political affiliation with a specific opinion, they know of a new question they can ask someone. That new question should generally reveal what your political party is most likely and they can then proceed with that "most likely" answer.
So "who cares" is those who cannot directly ask you something. They will ask you something else and use that answer to deduce the information they cannot obtain directly.
If you want to see what various license plates look like from all of North America (as well as what they looked like in the past), there is a website that with a short URL. Strangely enough, the website never gone to https. The link is http://www.15q.net/