One extra food in the center tile of every city that's on a river or a lake, in despotism. Half-price aqueducts for those that aren't. One extra food in the center tile of every city once out of despotism. The ability to irrigate deserts to get 2-food tiles out of them before rails (and three after).
Long story short: more food, faster growth. Food=population=just about everything, in civ.