There are states with ‘closed primaries’, where certain elections are only open to registered party voters - ie a Democrat would not be allowed to vote in the Republican primary and vice-versa.
This, and ‘because that’s how we’ve always done it’ are probably the main reasons party affiliation is part of voter registration.
This, and ‘because that’s how we’ve always done it’ are probably the main reasons party affiliation is part of voter registration.