The Harrier is in service with the United States Marine Corps (215 aircraft), Italian Navy (18 aircraft) and the Spanish Navy (17 aircraft). Italy and Spain are using the Harrier from their aircraft carriers, the USMC from Amphibian ships, NOT aircraft carriers (standard not, but they can operate from a/c carriers).

United States Marines Corps
The AV-8 Harrier was developed for the United States Marine Corps (USMC) for the attack and destroy of surface targets under day and night in visual environments. The first aircraft - AV-8A Harriers - came in service with the USMC in 1985. In 1995, the AV-8B Harrier II plus standard was introduced to replace the AV-8A and A-4M aircraft. .
The USMC had a need for tis aircraft: to could create a larger tactical mobility, for quick response, reduce of operation costs and greater bases flexibility, to operate from sea and from land, they're useful for special combat and expeditionary requirements.

Technical