2. Minimize distance between points. Putting everything on a line means increases the distance for point to point trips. In a 2d city, things are closer together. In a 3d city, even more so. This affects travel, networks, etc. Important / highly frequented things will cluster in the middle.

higher dimensions also increases the length public infrastructure laid down and number of routes. connecting a 5x5 grid needs twice as as many lines as a 1x25 grid.