The correct answer to this open question is the following.
The executive branch has agencies such as the FBI and the EPA because as the Head of the State, the President of the United States is the most important figure in the country and is responsible for conducting the nation in the best way possible. The President has the power to implement and enforce the kind of legislation and programs that benefit the people. That is why he has a group of advisors in the White House and many Departments such as the Treasury, Defense, Agriculture, the Interior among many others, to put these programs in action. He also oversees the work of agencies such as CIA, FBI, or EPA that were created for specific purposes for the benefit of the citizens.