The Smell lets you "leave the thought", literally, in the sense that if you are in a square and you write a public message on it, that message will belongs to the place where it was left and not only to who wrote it. Then the messages are indeed a landlord, who is hiding behind a nickname, but belong to the places in which they are written. And to read or write a public message in a certain place you need to be physically there.
So, if you are in 67^ in New York and write a message on Smell, to see that message you must be in 67^ in New York and there is no others way to see the message. For another square is the same. So anyone can read the messages left by others only if they are in the same place where the message was written. You can see it as a way to comment on the world, as a way to denounce something, to leave a reminder pegged at some place or just a way to "hear" the voice of other people before you have been where you are at that precise moment.