If keyboards had these button by default, this would actually allow the user interfaces to be cleaned up and have a lot of these 'non-metaphorical' UI component

If keyboards had these button by default, this would actually allow the user interfaces to be cleaned up and have a lot of these 'non-metaphorical' UI componentes removed - "closing a window", what is this a metaphor of? When I close a real window, it doesn't vanish, it just stops air getting inside!