You don't register your party unless you want to vote in that parties primary election. This is where a political party decides which of their candidates should

You don't register your party unless you want to vote in that parties primary election. This is where a political party decides which of their candidates should actually run in the real election. Most people just vote in the general election where this isn't necessary. The reason the Democrats or Republican might want voters to register for their party primary is to avoid the other party getting their weakest primary candidate the win so that they can beat them in the actual election. Also keep in mind that there is no federal election, so when you hear people talk about "the popular vote" it's a misnomer. There isn't actually a national popular vote. All elections are run by individual states, and in fact even national offices like President can have different candidates on the ballot in different states, depending on what happened earlier in the election season during the party primaries.