It's not a shape cut into the back, it's a "bridge." It's a removable piece that holds the jewels for the pivots. It makes assembly easier than if the whole back plate had be installed as a single piece. The shape is determined mostly by mechanical requirements, and then it's jazzed up a little for aesthetic reasons. The "cities" are the jewels.
Here's a picture of bridges on an all-mechanical watch. Same thing.
