You may not know this, but there are almost 100 political parties that run for government positions every year. Of course, the Democrats and the Republicans are the most popular political parties that usually gain all of the votes. Like the two main parties, third-parties are also built on a foundation of idealologies and opinions of one person or one group. However, most times these third-parties do not get very far and have trouble making it at the national level.
But don't take that as a reason that third-parties are any weaker. We all remember H. Ross Perot and his third-party candidacy in a political party he formed called the Reform Party. The Reform Party is still around today, but is not a speck of what it used to be. In the latest presidential election, the Reform Party could not even gain more than 500 votes. But when Perot was heading the charge in the party's first election in 1996 (Perot ran as just an Independent in 1992), they received over 8,000,000 votes, garnering 8.4% of the vote, which is amazing for third-party candidacy in general and shows that a third-party can do it with the right candidate and platform.
Other well known third-parties in the United States include the Libertarian Party (who just nominated Former New Mexico Governor Gary Johnson for the upcoming election), the Socialist Party (who nominated Stuart Alexander), and the Green Party (who have not chosen a candidate). Historically, some third-party candidates have had success in the general election: Theodore Roosevelt (1912- Bull-Moose Progressive), Ed Clark (1980- Libertarian), Ron Paul (1988- Libertarian), H. Ross Perot (1992- Independent; 1996- Reform), and Ralph Nader (1992-2008 Green/Independent). Many claim that the presence of both Perot and Nader, on the 1992 and 2000 presidential tickets respectfully, shaped both elections.
So for anybody who says that third-parties do not matter, they do. A third-party can easily take votes away from one party and block them out of the White House. Maybe one day, a third-party candidate will rise up to win the election. But until then, all a third-party can do is transmit their views to the public and play spoiler.