I assume it has something to do with registering to vote at the DMV (a service they offer). I always naively thought they offered to do this because it was just the government trying to do something that benefits the people (their job), but now we know it's because that is very valuable information that they can sell along with your other information (otherwise they may as well toss it after forwarding it to the appropriate elections registration desk).

If they're going to store it for any reason, they should at least hide it from the low level workers... there's no reason anyone, of any political affiliation, should have to even talk about something so irrelevant to the role of the DMV. Not to mention, it would speed things along for everyone and that's a real benefit that we all need.

Political party affiliations are set during voter registration, which in some states can occur at the same time as getting a driver's license. (Setting a party affiliation gets you a ballot for that party's primaries.) So it'd all be in the same DMV form and the end result is a drivers license and registered voter. Party affiliation is generally public information (search for it).