I think it had two purposes: one, it helped the reader visualize the circles of hell and easily recall who-resided-in-which circle. Furthermore, granting it "map" status makes hell and its regions seem more like a "real" place. Travel here, travel there, longitude, latitudes, and so forth. What sin are you particularly guilty of? Well, check out the fine accomodations you can expect upon arrival! Meet your room-mates! For eternity!
Just as a reminder, the levels, from top to bottom and their inhabitants are as follows:
Circle One : Limbo; Circle Two: Lust; Circle Three: Gluttons; Circle Four: Hoarders; Circle Five: The Wrathful; Circle Six: Heretics; Circle Seven: Violent; Circle Eight: Fraud, and finally, Circle Nine: Treachery
There is a really cool interactive map at the link below, and also an interesting article about Dante's Hell. A link to the main eNotes pages listed too. Good luck in Hell! :)