This isn't a minimize all button. This is a button to push the windows like inventory, appearance, etc. to the background until they are needed again.
Sometimes I have lots of windows open (mostly inventory) and I quickly have to mess with a HUD for 3 seconds or interact with something in world. A feature like this would allow me to quickly hide all the windows, interact with the world or HUDs, then toggle back to the original arrangement as if nothing happened. Currently, we have to minimize each window one at a time until we can see, then restore them individually again.