To add to this: if the opposition party consistently shows up to vote, the dominant party gets nervous, and has to focus on the chance of losing. Not showing up means they’ve truly won.

It also shows the opposition party that they can and should invest the time in supporting that area, because there’s people who haven’t given up yet.

Also, the president isn’t the only person on the ballot, and small races are where more radical third parties actually have a shot!