The three roles of Congress are to pass laws, manage collecting and spending of Federal moneys, and shaping foreign policies. In their position as law makers, the Constitution refers to Congress as the legislative branch that pass and veto all laws that come to the Federal level.
Considered the most important power, the "power of the purse" allows for Congress to set the spending and taxing policies of the nation. Determining the national budget, setting outlines for taxation and the subsequent spending of these moneys is a monumental responsibility and Congress decides the allocation for nearly $2 trillion US dollars every year. Born out of the colonial fears of "taxation without representation," the writers of the Constitution took the care to give ultimate deciding power of taxation to the Legislative Branch, not the Executive Branch, feeling that the Legislative Branch is closer to the peoples of the nation. James Madison in the Federalist papers called it "the most complete and effectual weapon with which any constitution can arm the immediate representatives of the people". It checks the power of the President and gives Congress vast influence over American society.
Congress also shapes foreign policies by holding the power to declare war. In the Constitution, Congress was given more powers over foreign policy than the Executive Branch. Congress not only declares war, but also ratifies treaties made by the President and budgets Presidential spending in foreign nations. Congress also has the power to start investigations against the Executive Branch under suspicion of abuse of power and often has the authority to appoint the heads of the Executive Branch.
Montesquieu's tripartite system allows for Congress to have specific powers for themselves and also powers that enable them to hold the Executive Branch in check.