We could learn a great deal. For example, we might discover secrets about the past inhabitants of a place. Their joys, sorrows, and daily routines. Maybe we'd find out about long - forgotten romances or family feuds. The walls could reveal the evolution of a building, from its construction to any renovations over the years.
Our perception would change drastically. We'd no longer look at historical places as just relics of the past. We'd start to imagine the real people who lived there, their emotions, and their experiences. It would make history more tangible and relatable.
Perhaps the uncertain walls are a symbol of the changing relationships within the city. They could represent how the people in the city interact with each other, with the walls acting as a metaphor for the barriers or connections between individuals or groups that are constantly evolving.
One reason could be homelessness. If a person has no place to stay, they might find a hidden space in the walls of an abandoned building as a form of shelter. Another reason could be mental illness. Some people with certain mental disorders may feel the need to hide or isolate themselves in such an extreme way.
Rayleigh would be a great candidate. He's an experienced pirate and has already trained Luffy for a while. He has a lot of knowledge about the world of pirates and could give Luffy a talk about the deeper secrets of the Grand Line or how to deal with the powerful enemies that lie ahead.