You can also set it, if built in coastal cities, or those within one tile of the coast, to "increase food in water". For a coastal city, that can give a big boost in production. Right now, I do that with the granary, and a few other things. I do this as aside from irrigation, there is no way to increase food production from land tiles. I find that most irritating, as one of the areas that I study is the increase in agriculture yields over time.