1. Even if a candidate doesn't win it doesn't mean voting for them doesn't influence the political landscape. The more people who vote for a third party the more votes the other parties didn't get, and they want to tap into that. It isn't the majority who wins elections, but rather the independent voters who can turn that 49% into a 51%. Voting with your conscience tells the candidates of major parties what they should do to win you over in the future.
2. You don't like the other candidates anyway. Sure, you can say that one of them is the lesser of two evils, but they are still pretty damn evil. Why endorse that?
3. The current parties are on their dying breaths. Look at how devasted the GOP is by Trump, and unless they change drastically they are in for a demographic problem 10-20 years from now. As for the Democrats, you might not know this because the current president has a D next to his name, but they are in a very risky situation right now. If Republicans win the presidency and keep hold of the house, then Republicans effectively control all three branches of government on almost every level. The Democratic party needs to win this election to survive. Not to mention they have their own schism in their party between progressives and moderates. A third party can very well become a second party in the next fifty years or so. Political change doesn't happen in a night. But it won't get there if you keep voting for shitty politicians
4. Getting more than 5% votes alleviates the pressure of getting ballot access through petition and allows a third party to spend money on other things. Getting 15% opens up the third party to debate in the next election. The benefits of each additional vote does not scale linearly.
5. The president doesn't really do much. The powers of a president are limited, and the amount of influence you have over their election is miniscule. Showing what your beliefs really are affects a lot more, and can lead to more change on the state, local, and national legislatures - who do make the big changes. While it might seem like a big deal in the short term, it really is not. In the long-term the same things end up happening if you keep voting for the lesser of two evils.
