He has the power of the Executive Branch vested in him. He acts as head of state and commander in chief of the armed forces and is responsible for implementing and enforcing the laws written by the legislative branch or Congress. These executive powers also allow him to appoint heads of the federal agencies, including the Cabinet.
In short, it’s no easy task to be president of this country and the position holds a tremendous amount of clout and prestige. It is presumed that the person holding the position is capable of carrying out these duties, as well.