Why 'utility destroyer' is not a Royal Navy classification

Why are the Royal Navy's alphabet-class destroyers sometimes called utility destroyers?

That is a later historian's label; the Royal Navy itself classified them as fleet destroyers, alongside large fleet, war-emergency and escort types.

The term utility destroyer appears in recent writing, but not in Royal Navy usage. The service called its general-purpose combatants fleet destroyers. Larger ships such as the Tribals and Battles could be described as large fleet destroyers, while wartime simplified designs became war-emergency destroyers. Escort destroyers formed the other functional category, so the utility label is best understood as a later analytical convenience rather than contemporary doctrine.