Right, the voting system is bad. I don’t like it, either. However, if the dominating candidates are Biden and Trump, and you voted for lesser Democrat candidate, then Biden doesn’t get your vote. If Biden gets too little votes, then your next president is Trump. You wouldn’t have chosen it explicitly, but it is your implicit vote.
That being said, if Biden has some strong competition and another candidate is appearing favorable, then it makes sense to vote for them. Voting for someone you know is going to lose is just acting in principle without making any impact on the election.