When a warship becomes a cargo carrier

Is there ever a case where using a warship to carry cargo is advantageous?

Yes; warships can be justified for compact, extremely valuable cargo that must get through quickly and securely.

A warship is normally an expensive and inefficient cargo vessel. The tradeoff changes for high-value, low-volume loads that must arrive despite enemy action. During the Second World War, fast and protected naval ships could be assigned such cargo because they had a better chance of forcing passage than an ordinary merchantman. Gold is an awkward example because it is extremely dense, but it still demonstrates the underlying rule: strategic value can outweigh cargo efficiency.