It gives the game a powerful sense of direction, a purpose. You will either try to destroy the world (make the AC high) or save it (low AC). OTOH, in some games, the AC won't budge very much (you still occasionally get a world with mostly two religions, Fellowship of Leaves (FoL) and Runes of Kilmorph (RoK) in which case, the AC will usually rise only when a city is razed or a good civ destroyed).