It’s because voting is an intrinsic good, not merely an instrumental good.
In other words, even independent of your vote’s affect on the outcome of the election, voting is a good thing that matters in itself.
This is because when you vote, you are exercising your rights, and doing so for the good of the nation. Exercising your rights, when done for the good of others, is a reflection of the image of God and his plan for human government.
Therefore, voting is good in itself and there is much more going on here than simply determining the outcome of the election. In one sense, voting is an expression of what it means to be human — namely, the fact that we are all equal before God and the law and that God put us on earth not to be tyrannized over by government, but rather for government to be the servant of the people.
So, if you haven’t yet, go vote! And do so even if you think the outcome for your state is already set.