According to CliifsNotes, the main functions of the two major political parties are to represent interest groups, represent constituents, simplify choices and make policy. In general, the goal of political parties is to make the legislative process simpler and more responsive to the will of the people. The two parties also give citizens a common ground to group together and attempt to create political change.
According to Scholastic, in the United States, the party system is generally referred to as a two-party system because it has been the case that only two parties are consistently large and powerful enough to influence national elections: Democrats and Republicans. Other parties do exist, but it is rare that any gain enough of a foothold to elect candidates to office at the national level.
Scholastic explains that the Democratic and Republican parties are highly organized, and this organization helps them to function better. The two parties have local, county and state committees, as well as a national committee with one man and one woman from each state. These different levels within the organizations help the parties to keep in touch with state and national politics and with the will of individual citizens, whose interests they are supposed to represent.
CliffsNotes states that political parties also simplify political choices. In an attempt to reach as broad an audience as possible, parties simplify their positions with broad philosophical platform choices. While many issues are complex, it usually boils down to Republicans on one side and Democrats on the other, making it easier for citizens to pick the side they agree with rather than needing to research individual politicians.
Political parties also set policy by putting their platform into legislation when possible, says CliffsNotes. When the Republicans are in power, they try to advance legislation that they support. The same goes for Democrats.