I cannot think of a legitimate or legal reason.
Hypothetical bad reasons?
Sharing with labor union political villains comes to mind. Others?
“a legitimate or legal reason.” —> I’ll hazard a guess that it is for election reporting. It allows them to do all sorts of statistical analyses comparing actual vote tallies to registered voters.
Or for target marketing.
Im pretty sure its available to any campaign or news organizations who pay for it. I worked on a campaign and ordered a copy for the legitimate purpose of knocking on doors on behalf of the candidate. IIRC it was the county that provided it, and there were some rules on dissemination, but not retention.