Why do Angel Food Cakes have to be turned upside-down while cooling?

According to the experts we contacted: "Angel Food Cakes are really nothing more than expanded egg white foam with sugar added for sweetness, and flour added to stablize the foam and prevent it from collapsing during baking and cooling. Due to their inherent weakness, angel food cakes would collapse during baking and cooling if it weren't for two things. First, an angel food cake pan is never greased. This allows the batter to grip the pan sides for added support. The cakes are then stuck tightly enough to the pan after baking to allow them to be inverted without the cake falling out of the pan. By inverting the pan, we prevent the cake from further settling during cooling and obtain a light, tender finished cake that has a chance to lose the extra moisture to the atmosphere and form a skin."
http://foodpluspolitics.com/2009/02/09/about-angel-food-cake