Why waste a vote on Donald Trump or Joe Biden?

I’ve decided to waste my vote in this election, and I’m not going to change my mind.
I’ve been told by Democrats that I’m essentially voting for Donald Trump, and by Republicans that I’m voting for Joe Biden.
“A vote for a third party is a vote for the opposition,” they say. This, of course, is implying that I would have voted for their favored candidate otherwise. But this isn’t the case, because if I didn’t vote for a third party, I would n