FM Broadcast started out Horizontally polarized. Circular polarization was later authorized under 2 stipulations. The vertical plane power can not exceed the horizontal power ERP, and the horizontal plane ERP is used for the stations ERP. So a station with a horizontal antenna can effectively double the ERP by switching to a circular antenna, of course it takes 2 CP antennas to equal one horizontal antenna.
On the receive side of things, Horizontal is usually used in homes and Vertical or CP on cars. CP also helps quite a bit with mobile flutter.
