I've never understood "urban agriculture". Cities have one of the highest land prices due to their proximity to people/commercial centers, I don't know why turn

I've never understood "urban agriculture". Cities have one of the highest land prices due to their proximity to people/commercial centers, I don't know why turning that into a farm, which takes tons of land, makes sense. On the other hand, I'm very interested in studying the very long-range evolution of cities (500+ years). It fascinates me that Detroit was kind of this pre-Silicon Valley area with high concentration of a single industry and tons of wealth, and after that's left, what happens now? How does a city reconfigure itself when its tax base drops by, say, 50%? How does it deal with its pension obligations, massive infrastructure bill from all the now-not-needed roads, parks, water pipes, city lights, and buildings? In short, how does a city "scale down" in a way that isn't damaging to its institutions or retirees?