The UK flies the Harrier because they have STOBAR carriers, but why does the United States Marine Corps fly the Harrier?
$\begingroup$ They also fly F-18's, which are not VTOL (they also fly F-35's) and other non-fighter aircraft. $\endgroup$– Ron BeyerAug 29, 2019 at 21:39
1$\begingroup$ Forward deployed close air support where long runways aren't available. $\endgroup$– Michael HallAug 29, 2019 at 21:45
$\begingroup$ At the time this question is asked the UK has not operated Harriers for eight years, has only one commissioned aircraft carrier, and that uses a STOVL configuration. When the Royal Navy did operate Harriers they flew from carriers that also used the STOVL configuration. $\endgroup$– CatchAsCatchCanAug 29, 2019 at 22:36
$\begingroup$ Perhaps because the USMC doesn't have any aircraft carriers? Not to mention that carriers aren't much use in places like Afganistan :-) $\endgroup$– jamesqfAug 30, 2019 at 5:17
Because when selecting aircraft the requirements were for close air-support and to be able to operate from amphibious assault ships like the USS Tarawa and the USS Nassua.
These ships have very small decks that aren't geared towards launching larger aircraft like the F-18.