Not that we have that many buildings of notable height, but when you do achieve some altitude downtown, there’s nothing going on on any of the roof real-estate you see below you. If you want to be a real city, you need density. If you want density, you need height. And if you want height, you might as well make livable or otherwise useful space all the way up, including the top of a building. Greenery on top of a building is not only a smart move environmentally, but its essential for human living in the city, who need as much nature in their urban area as possible.