Navies needed rifles because sailors fought ashore too

Why did the Royal Navy need so many firearms in World War I?

Boarding parties, shore parties, port security, and rapid-reaction land deployments all required rifles and small arms.

Boarding and shore parties created a regular need for naval firearms. Port security and emergency land deployments also drew on ship crews. The Boxer Rebellion provides an example of sailors serving as a naval rapid-reaction force ashore. Naval personnel therefore had to be prepared to act as armed detachments as well as shipboard specialists.