Voting third party expresses your dissatisfaction with the major parties and helps to legitimize the third party in the eyes of others so it can have a chance in the future.

but only if the outcome is assured. in an election where there’s any chance of your vote making a difference, voting third-party is essentially a vote for whoever wins. in a first-past-the-post system, if you vote third party it means you could have voted against the winner, but chose not to