Green walls, green corridors, sky gardens, etc., can also be integrated into a dense urban landscape. Even an old, pre-WWII building can have green plants on its roof. Greenery provides fresh air to citizens. It absorbs carbon dioxide and releases oxygen and water vapour. On the micro scale, it can alleviate the urban heat island effect and on the macro scale, it can reduce greenhouse gas emissions.
I think that greenery in inner cities makes the environment look much more nicer. Plus, more greenery means more CO2 storage. More greenery in inner cities makes it look much less like a concrete jungle, and provides a much more peaceful living place. It's good to have flowers outside your window.