Assuming we're talking about a two-party system in a winner-takes-all electoral system, people join/vote/support third parties for the following reasons:
- The two major parties are not responsive to voter needs, whether through their politicians, platforms, or political priorities. Some historical examples: Reform Party, Progressive Party, Populist Party, Socialist Party.
- A third party's platform aligns with a voter's preferences more than either major party.
- A voter might vote for a third party as a "protest vote," meaning that under circumstances where the voter is displeased with both major parties, a voter will seriously consider voting for a third party to either send a message of non-support to both major parties or to force a change in one of the major parties by depriving that party of votes.