Circles are the simplest shape for our eyes and brain to understand. When faced with a swathe of circles, we're not as overwhelmed by the information. It's easy to look at, and easy to read.
Neighborhoods don't have corners: When you're in-between three different neighborhoods, which neighborhood are you in? There are no-man's lands all over a city.
Not only do circles create a contained space, but they also imply entire worldliness. In old times, a village was often mapped in a circle, which was probably the whole world to its inhabitants.