And you can say the same in reverse: the benefit to having two small cities instead of one large city is that you can have twice the buildings! Thus you can have two sets of buildings creating culture and happiness, and they quickly cancel out the extra unhappy faces you get from an extra city so (if you keep the total population the same) you're left with many more happy. They also potentially create more culture than the higher threshold an extra city puts on the next policy.
You work two sets of city tiles, so you can get access to more resources.
The tendency will be for the cities to grow to where their combined population is greater than one could get to = more science, and more cash from gold tiles.
You also get the opportunity to produce things simultaneously in two cities. For example, you can purchase two units a turn in two cities not just one. You can be building two wonders at the same time, not one after the other and risk missing one.