Same question about why the US's navy marines fought in the deserts of Iraq and Afghanistan far from any beaches, rather than leaving the land warfare to the US army.
I met a mechanic working on a jet engine pulled from a fighter in the repair facility of a US aircraft carrier (his biceps were as thick as my legs); I asked him why he was wearing a Marine pattern camouflage uniform. He explained that he was a Marine just on loan to the Navy because they needed an additional jet mechanic on the ship.
He told me that unlike the Navy sailors he had his gear bag with him and was ready to be dropped ashore to engage in hostilities at a moments notice.