Why do we need a Secretary of Defense when we have a Secretary over each of our Military Branches of Service?

While the SECDEF does oversee all branches he does not only account for each Service Component Command. He/She is in charge of all the Unified Commands and Defense Agencies. He/She also writes the National Defense Strategy annually. Without the SECDEF we would have no joint unity within the Armed Forces community. There is the CJCS, but their job is more in regards to policies rather than command and control.

Until 1948, there were separate Departments, with their own cabinet level secretary. The Secretary of War oversaw the Army and its Air Corps and the Secretary of the Navy oversaw the Navy and the Marine Corps. Creating a Department of Defense (and a Joint Chiefs of Staff) happened in the unification struggles post WWII (when the proponents of air power wanted to do away with the Naval Air and turn the Navy into essentially the Merchant Marine, get rid of the Marine Corps, cut the Army to a level to be occupation troops after the Air Force had won the war by nuking every enemy in sight). SECDEF was supposed to handle stuff that needed to be uniform--supply, comm, etc. Of course, bureaucracy being what it is, DOD became a mirror image of the old Departments of War and Navy and the new Department of the Air Force. The worst aspect of this unification/reorganization was that the Service Chiefs could no longer meet directly with their boss, the Commander in Chief. Instead, they had to go through a political appointee. That meant that the President got the politically filtered information.

The individual SEC will rightfully so focus on thier branch, what is best for them. There has to be someone above them to balance what is best over all, someone to be the SME who helps the POTUS see the big picture...not just hear the branch specific needs, wants and way forward.

Just as you have a chief of staff for each service with a chairman of the joint chiefs of staff you need one to break the chain

Why have a Command Sergeant Major when you have a First Sergeant at each of the Companies?

The term "span on control" comes to mind. Does anyone think that a single person could manage something as huge as our DOD?

First off, the USCG does not have and has never had its own, personal, private secretary. Second, I say dump the service secretaries and keep the SecDef. Start with Ray Mabus!!!