The party affiliation data in the voter roles is probably one of the most heavily used fields in there to boot. A lot of petitions need to be signed by a certain number of registered party members -- the local party apparatus has to get signatures each (election) year to demonstrate that they are indeed the local apparatus for that party, individuals wanting to primary need a certain number of signatures to get on the primary ballot, you may also need to collect signatures to force an open (write-in) primary with no specified candidates...

Campaigns will also use it to target their literature; you might send one mailer to the members of your own party ("X is a totally loyal party member, here's seven things he did to support our core party values!"), another mailer to the opposition party ("X is not a total baby eater, here's five ways he crossed party lines to support things you probably care about!"), and a spattering of other mailers to the third-parties to emphasize support for their particular interests.

The weirder thing to me is that these databases also include birthdays. You could send birthday cards to everyone.