Another very good question, reader Will. Heat penetration is the answer. Without that center hole, an angel food cake would be an extremely broad and thick mass. Heat from the oven would have a hard time reaching the center before the outside over-baked. Meringue-topped pies are a good illustration of this problem. Big as they are, the very centers are often under-baked or weepy, because it’s hard to get that middle region hot enough without overheating and breaking the rest of the meringue.
The importance of center heating can also be seen in cake-style doughnuts. They start as a dense batter and have to fry up fast in a very heat-intensive environment. The hole in the middle allows them to cook evenly, from the outside inward and from the center outward, so there are no under-done spots in the middle.
It’s a popular myth that the center tube gives an angel food cake more surface area to “grip” as it rises. The reality is that cakes don’t “hold on” to pan sides, nor do they receive “support” from pan walls other than to simply be contained by them. For cake layers the name of the game is heat. The quicker it penetrates the mass of batter all the way through to the center, the more evenly the cake bakes. Pan walls facilitate that heat transfer, which is why batter near pan surfaces rises and sets faster than the batter that’s further away. Great question, Will!