1) I commend you if you are taking the time to do this. I know how time consuming building maps, (particularly to scale), in Minecraft can be......

2) It's probably purely decorative. If you look around the city, and particularly off the edges, there are A LOT of areas we can't reach that seem like they would be interesting to explore....

3) If you're asking as to the purpose in terms of theoretically structuring it as a real city, then I would guess it's a form of balcony/walkway surrounding the building in question, both allowing access to different rooms of the building that they don't want connected inside, and allowing you to get some fresh air from whatever rooms it connects.....