Why unprotected cruisers belonged outside the battle line

Why build an unarmoured vessel and call it a warship if it would be outclassed in combat?

Unprotected cruisers filled necessary low-cost policing, patrol, and presence roles much like later sloops or modern offshore patrol vessels, but they were not meant to fight major warships.

Unprotected cruisers were forerunners of the later sloop: numerous, inexpensive ships for peacetime duties that did not justify the time and value of a major combatant. Modern River-class offshore patrol vessels illustrate the same division of labor, because necessary constabulary work does not require a frigate, destroyer, or aircraft carrier. Sending a high-value combatant instead can consume readiness and expose a complex ship to avoidable damage. A warship's value includes its trained crew, institutional experience, weapons, sensors, command systems, and deployability, not merely its construction cost.