It's good point about the amount of sunlight available via the roof being limited. The main value I see comes from having a controlled environment. I guess it doesn't really need to be vertical, but the key part is that you can regulate temperature, humidity, and so on. And yeah, I think this sort of stuff becomes much more interesting when the profit motive isn't the primary goal. I imagine China is investing into this as a way to ensure stable food supply for the people as the primary goal.