American political parties are essentially representative groups that are advocates for your personal beliefs. There are two major parties currently, Democratic and Republican which have won every election since 1852. Citizens are obliged to cast their votes for leaders that they feel closely resemble their desires for growing the company. The two party system is designed to make sure no one party gets too powerful and to help the country grow in the direction the populace wants.
It isn't perfect by any means but for governing such a large body it has been effective thus far, but could use improvement.