A strong defensive emplacement (such as a fortress) provides some advantages, even to a mobile army. If your opponent chooses not to attack it, he has granted you the initiative -- you can move your forces out to attack his at the time of your choosing.

Of course, a battleship, being mobile, does not suffer from the disadvantage of being easy for your opponent to bypass. And a battleship, carrying a stupidly large armament, has the advantage of being able to blow up lots of your opponent's stuff. Its primary disadvantage is its expense, which is why you don't see fleets consisting of only battleships.