And which one of those is going to require the Marines assault a defended beachhead? Which is a situation where air power cannot be employed to greater benefit?
Did you miss the part where the Commandant of the Marines says that even their VMF and VMA squadrons can get weathered in?
They aren't just the big guns...they serve as TLAM launch platforms as well.
They could be a launch site for Predator and other Armed UAV's.
The guns on a Battle Wagon have never just supported Navy and Marine assets.