The answer leans more towards it just being because they were available manpower. During the age of sail, the Royal Marines had been expected to provide manpower towards part of a ship's armament where necessary. However, they were first and foremost soldiers - providing boarding parties and landing parties, or marksmen in battle. In 1804, the Royal Marine Artillery (RMA) was formed to man the armament of particular ships (the older marines were designated the Royal Marine Light Infantry, RMLI). They manned the guns on ships like bomb ketches (ships armed with mortars). This was a more subsidiary role, on ships designed for coastal bombardment.

However, as technology progressed, and as the Royal Navy grew, the Marines began to specialise in naval gunnery, Technology meant that manning guns became a more specialised business, and one requiring much more hard labour. The Marines met this by building a cadre of specialised gunnery experts, especially amongst the RMA, which established gunnery training batteries at its HQ at Eastney. As the RN expanded, it quickly became clear that it would not be possible to meet the manning requirements without drawing on new sources of manpower. To solve this captains were instructed to train Marines in gunnery, and to use them to man a proportion of their armament. The RMA was typically used to man the main armament, while the RMLI manned the secondary armament.

In 1903, the Royal Marine Band, which had traditionally been stretcher bearers and members of damage control parties, received a new duty. They were to man the new fire control equipment that was being fit to battleships and cruisers. The RMA and RMLI manned one turret aboard the Dreadnought battlecruisers and battleships of WWI, but they were merged in 1923. In the following years, the Royal Marines added anti-aircraft gunnery to their list of skills. During WWII, they manned one turret and a portion of the secondary armament on battleships and cruisers.