Why are third parties important in a political system?

In the U.S., "third party" refers to any party other than the major two, which are the Democratic and Republican parties at present. Third parties have sometimes launched large campaigns and have won public offices, but have not obtained significant or consistent representation in the federal government. Barriers to third party success include a winner-take-all election format, ballot access laws, debate rules, and the vast resources of the two major parties.