Why battleships carried 15-inch shrapnel shells

What would a 15-inch shrapnel shell do to a destroyer, and why did the Royal Navy carry one?

Destroyers were among the intended targets, and the ammunition also proved useful for supporting land operations.

The Royal Navy's First World War 15-inch ammunition is described as including APC, CPC, high-explosive, and shrapnel shells. A question about the effect on a destroyer leads to the explanation that light targets were part of the purpose. The same ammunition could support troops ashore. Queen Elizabeth's use of shrapnel at Gallipoli is recalled with an explicit qualification, while the imagined Ottoman complaint about losing a battalion is a joke rather than a casualty figure.