Converting a merchant hull into a missile arsenal ship

Why might a navy convert an oil tanker into a missile arsenal ship?

It offers a cheap large hull, but a container ship is usually easier; a tanker is most plausible when it is the available option or when a disguised arsenal ship is desired.

A merchant conversion provides a very large hull at comparatively low cost. A container ship would be easier to adapt than an oil tanker because its layout is already suited to modular cargo spaces. A tanker becomes plausible when no better hull is available or when a navy wants the ship to retain the outward appearance of an ordinary tanker. In that concealed version, deck piping could mask covers over vertical-launch cells, trading transparency and specialization for surprise and missile capacity.