Cheap transportation means more freedom of movement which in turn makes for greatly increased trade and commercial activity. When it’s cheap to get anywhere in the city, everyone benefits on every single level. China is rather unique in having actual population density that makes these investments meaningful. Like virtually all infrastructure projects in China. They are derided for being underused, and then a few months later they are at capacity and become critical components of life for millions. Of course, that part of the story, the one where idiots admit they were completely wrong, never makes it into the news. Hack “economists” have to keep pretending they know what the hell they are talking about I guess.