How important is party affiliation in America?

I'll not say which one, but used to belong to a third party. I used to think that I was a benefit to this party because they could count me as a member. I became unsure how beneficial I was and joined the democratic party in order to vote in their last primary. Now, I plan to join the republican party for the same reason. So, how much do parties benefit from membership statistics? What is more beneficial? Being counted as a member of a third party or taking positions that let me vote on which direction the two major parties will take?