In the U.S.A., Why Do People (Need to?) Register as Republican or Democratic (or Independent?)?

[Northern_Piper]

Previous thread on that topic:

Could the Democratic Party have kept Bernie off the primaries?

This point has come up in various permutations over the past few months, and as a furriner, I’m confused. Over in the “Bernie Withdraws” thread, Left Hand of Dorkness says this: But I thought, from previous threads on primaries, that access to the ballot for primary elections is determined by state laws? And if you as a candidate meet the state’s requirements to enter a primary, the party can’t keep you from entering? And, since those state laws are set by the 50 states, there are 50 differ…