The State Department is the Executive Agency responsible for America's foreign policy. As such, they are responsible for handling international matters and do so through diplomacy and the work of Ambassadors in countries around the world.
A good Secretary of State must have an excellent command of foreign policy and the United States' interest abroad. They must also be an excellent manager, as the Secretary of State is responsible for a vast agency. I would think that these would be good skills for a President as well.
