Why does Angel Food Cake use a tube pan?

Structure is the main reason a tube pan is used for angel food cake. Angel food cake rises a lot, but does not have much of any gluten network or other means of supporting this structure. The egg whites can hold the air bubbles initially, but will lose them eventually. (Hence you should not delay baking after the batter is mixed, and you should treat the batter gently, spooning into the pan and taking care to not slam the pan around.) The tube pan helps because as the batter rises, it can "climb" the pan, sticking to the edges. This is also why angel food cake is left to cool upside-down in the pan for an unusually long time; it should not be removed from the pan until it is completely set. If it weren't for the tube, the center of the cake would not have anything strong to hold it up, so it would collapse. This is not an issue for cupcakes because they are so small.