Why does the U.S. Marine Corps need the Judge Advocate Division?

If the U.S. Navy JAG Corps also provides services to the U.S. Marine Corps, why does the Marine Corps need their own Judge Advocate Division? The U.S. Navy JAG Corps protocol states that the billet for Judge Advocate General of the Navy can be filled by a Vice Admiral in the Navy or a Lieutenant General in the Marine Corps, while the highest ranking Officer in the U.S. Marine Corps Judge Advocate Division's highest ranking Officer is currently a Major General. If this is so, why does the Marine Corps need the Judge Advocate Division?