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

"One Secretary to rule them all, one Secretary to find them, one Secretary to bring them all and in the darkness bind them.

"In the Pentagon, where the Shadows lie."

(With apologies to J.R.R. Tolkein, but not necessarily Donald Rumsfeld)

In the profession of arms it speaks we work for the American government. This government is civilians. We advise those powers that be to do our job. Do I think we need one. Not necessarily but when we have a good one things got done.

Someone needs to be in charge to pull the branches together. It''s a matter of command structure.

To me it would be similar to asking why we should have a battalion commander since we already have commanders over each company. After all, they already lead their companies.