It's a matter of reach. The Navy can project national power directly to the enemy's shores, and with the advent of SSBNs, and SLCMs, into the enemy's heartland. The Army is all about winning battles on land, and even in the case of large and widespread campaigns (post D-Day, Korea, Vietnam), the focus is essentially on the battlefield, not overarching strategic concerns.Why is the Navy strategic and the Army tactical?
The one flaw I see in the current system is USAFs traditional disinterest in CAS, special ops, and battlefield transport, and their insistence that they know the Army's needs better than the Army itself does. They have consistently attempted to interfere in the Army's attempts to supply its own aviation needs (Apache, Comanche, Chinook, Caribou, Buffalo, etc, etc). I think all army cooperation functions with the exception of long range heavy transport should be given to the Army.
Sorry Biff, but that's my "grunt's eye" take on the matter.
Cheers,
Wes