Answer: Monitor the activities of Executive branches and programs.
Step-by-step explanation:
By the Constitution, Congress has the role of evaluating the actions and activities of the Executive's agencies and programs to ensure that they are for the good of the American people and not overstepping their power.
Congress does this through Committees that can indirectly or directly, investigate the activities of these agencies to find the information they need. They can even go so far as to invite the heads of executive agencies such as the CIA to brief them on activities.
This is a result of the system of Checks and Balances amongst the three branches of government to ensure that one branch does not abuse its powers.