Modern frigates emphasize defensive work, especially anti-submarine warfare, while destroyers have broader independent capability. An ASW frigate operating as a picket normally benefits from a destroyer nearby to supply air-defence coverage. The Royal Navy's Type 22 and Type 42 pairings during the Falklands War illustrate the logic: together the ships could address both air attack and the submarine concern around the fleet.
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How frigates and destroyers divide fleet protection
Modern frigates remain primarily defensive specialists, while destroyers add the air-defence and offensive capacity that lets a force or picket survive a broader threat set.