In a democracy, people rule, but an individual's voice is easily lost in a large and diverse nation, such as the United States. A way citizens can be sure the government hears their views is to organize themselves into groups. Once such group is a political party. A political party is a group of individuals with broad common interests who organize to nominate candidates for office, win elections, conduct government, and determine public policy. Most nations have one or more political parties. Of one-party systems, multiparty systems and two-party systems, only about a dozen nations have systems where only two parties compete for power. Although minor parties exist, two major parties dominate the government. These nations include the United States. In the United States, there are the Republican Party and the Democratic Party. Although many people would disagree, I believe this two-party political system helps keep the government of the United States strong.
Political parties mediate the relationship between citizens and their government. In democracies with competitive party systems, political parties pressure governments to respond to the needs and interests of broad segments of the population. With only two major political parties pressuring the government in the United States, the needs of the general public are looked at more carefully and with more interest. If many parties bring issues to the government, no matter how trivial, less attention will be paid to the major issues concerning the well being of the citizens of the United States.
Also, people like to know that their ideas and opinions are valued. In a political party, most of the members share the same ideas and opinions. With only two major parties, most of the opinions and ideas are very general. This makes it easier for United States citizens to feel they "fit in". With many political parties, it may appear difficult to relate with only one party.
Finally, the two-party system filters out the extreme elements that are present in other party systems. The multiparty system brings the extreme, and often destabilizing, elements into the government. It was not that long ago that the extreme Nazi party gained credibility by being brought into a German coalition government when none of the major parties could gain a clear electoral victory.
Although often put down and rarely appreciated, the two-party system gives the United States the ability to maintain political stability, a much sought after ideal in most of the world. It has been one of the key players of the stability that characterizes the tradition of American politics and continues to keep the United States government strong to the present day.