Why did capital-ship Marines carry towed artillery?

Why did Royal Marines aboard ships bring towed howitzers and other heavy equipment?

They might deploy ashore in strength and needed supporting firepower; capital-ship detachments could therefore carry surprisingly heavy equipment.

A capital ship's Marine contingent could form a substantial landing force and might need artillery ashore. Such detachments could accumulate equipment far heavier than a reader might associate with routine shipboard security. A story about a First World War Rolls-Royce armoured car aboard a Mediterranean Fleet battleship illustrates the point and explains why it would have been formidable against lightly equipped German paratroopers at Crete. The vehicle type had served with the Royal Naval Air Service, but how one later came under Royal Marine control aboard a capital ship is unclear. One Marine's recollection and an engineering officer's reference to dismantling a Rolls-Royce engine make the story plausible without proving the vehicle's identity. The uncertainty is explicit, and the anecdote is not presented as established fact.