It’s not for presidential races.

Its for state level races where you’re in one of the 40+ states where it’s a forgone conclusion what party wins the general.

So some people give up their presidential primary vote, to vote in the state level primaries for the party virtually guaranteed to win their state, then vote for their preferred party in the general even if their candidate won the primary for the other party

You might not think it’s common, but it’s the only way a lot of people’s votes have any actual effect, so lots of people do it