Political parties are nearly as old as the United States. The founders did not specify that parties would be part of the government, but the necessity to organize around competing ideas in order to win elections and govern led to their formation. Parties define and express a group's needs and wants in a way that the public and political system can understand. A party brings together various viewpoints on an issue and develops enough common ideas among enough people so that pressure can be brought to bear upon the political system. Parties also serve to recruit candidates for office and raise the money necessary to seek elected office.
This process usually results in two major parties in the United States, but does not always work perfectly. The appearance of strong third parties is usually a sign that the major parties have become unresponsive to the public.
A primer on the role of political parties:
http://www.democracyweb.org/multiparty/principles.php
