the key point is American political parties are a key component of our government.
The Constitution outlines the procedures that elected officials in all three branches must go through to exercise their powers. It tells congress how to make laws, it tells the president what he/she can and cannot do, and it outlines how the courts are to function. However, it says little about how we are to elect these officials. Political Parties are the "unofficial" institutions intended to fill that gap, and connect the voters with the head honchos in Washington.
If we didn't have parties, there would be no way to connect the voter with Washington, especially in a country this large.