it has some nice strategic value too. sa you just conquered an AI and you have all those crap cities producing nothing because of corruption, you could make them farmthingies: you limiet the size of the city and any food surplus goes into the national granary. All food in the national granary becomes tradeable on a 20turn base. Now imagine have a nice surplus of let's say 50 fpt you trade with a player at war. He buys your food, switches to an industrial economy. You than have with his food supply in your hands a tool to make him loose the war: if you cut his food supply, ghis people starve, reducing his production capabilities, if he swithes back to a more agraria economy, his production capabilities het lower too...