Commander in Chief has power to command the armed forces. The president has the ability to decide where to station his forces, where the ships will be, how weapons should be used. The commander in chief is the top persona in military and everyone reports to him for orders.
In this image George Bush is giving a speech to help inspire his troops while we are in the Iraq/Afghanistan war.
It is an important job for the president to be Commander in Chief because it brings unity to the troops. Even though there are four different parts of the military all being brought together under one ruler helps keep the unity and makes sure that people don't become seperated and unhealty competition breaking out between the different parts of the military. Also it's an important job because with all the sections of the military being under one person it's easier to communicate and get everyone sent where they need to go when they need to be there with no issues,