Plains Hills provide 1 Production in addition to the 1 Food and 1 Production yielded by basic Plains tiles. Plains Hills may be improved with Mines, which makes them significantly more productive than other Plains tiles. All Hills have a +3 Combat Strength modifier, which can strengthen city defenses, as well as protect vulnerable units.
Plains Hills (especially those on Rivers) are excellent locations to place cities early in the game. This is because cities automatically increase the yields on their tiles to 2 Food and 1 Production if either yield is lacking. This puts Plains Hills in the unique position of providing 2 Food and 2 Production when cities are placed on them. This improves a city's productivity early on, especially before technologies such as Apprenticeship and Industrialization greatly improve the yields of Mines. After this point, placing cities on Plains Hills makes less of a difference, and could even be discouraged as the Production yields of placing a Mine on one of these tiles outshines that of the city itself.
Hills in plains tend to be landmarks … just think of the highest point in Iowa, Hawkeye Point, at a stunning 1670 feet (510 meters). Given the flatness all about, these do give nice lines of sight for long distances over the endless, boring vistas.