system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When registering to vote in the US, why are you asked what party affiliation you would like to be associated with and does it matter?
gpt
Membership in a political party allows you to do things like vote in primaries. Election boards need this information to determine who can vote in what primaries. 

I would imagine it's also done at the request of political parties to ensure membership.