I mean first of all they want a list so they know you are in fact a citizen over 18 and also that you haven’t already voted.

Also the database includes logistical things like your mailing address for election related mail like mail-in-ballots, and your assigned voting location for states that do that.

Also it includes your party affiliation if you choose to list one so they know which primary you are going to participate in. It’s important to note the system of parties and primaries is somewhat informal because it’s technically not an official legal part of the system, it’s just people with similar political ideas organizing together to get their candidate a better chance.