If the state runs a partisan primary, they often want to know which party you're registered in so they can give you the correct ballot. Some state like mine pu

If the state runs a partisan primary, they often want to know which party you're registered in so they can give you the correct ballot. Some state like mine put everyone on the same ballot for the partisan primaries, and the instructions tell you to only vote in the column for your party, and if you vote in multiple parties your ballot is tossed out. Hence, we don't track which party you're "registered" as, because we don't need it - with the tradeoff of slightly more complex ballots .