Why modular warships carry their own cranes

Why do ships with containerized mission modules carry cranes when ports already have them?

Their own crane provides independence from damaged or absent port equipment and can serve other shipboard lifting tasks.

Shore cranes may be unavailable, damaged, or located far from the point where modules must be moved. A shipboard crane therefore preserves operational independence in dispersed or contested ports. It also allows modules to be handled at anchorages or improvised sites rather than only at a fully equipped naval base. Because many ships need lifting gear for boats and stores anyway, the crane is not solely a penalty imposed by modular payloads.