Why are egg whites useful for making angel food cake and sponge cake?

Egg whites are essential for angel food cake and sponge cake because they act as a primary leavening agent, allowing these unshortened cakes to rise without chemical additives.12

When whipped, the protein in egg white forms a voluminous, stable foam that traps air. This foam provides the light, airy structure characteristic of these cakes. Because the foam is delicate, it is often stabilized with an acidic ingredient like cream of tartar.231

During the baking process, the trapped air expands, and the protein structure sets to maintain the cake's volume. Because fats and lipids can cause these foams to collapse, it is critical to fold other ingredients, such as flour, into the mixture gently to preserve the foam's integrity. By relying on this mechanical incorporation of air rather than chemical leaveners, bakers can achieve the fine-grained, low-density texture that defines these delicate desserts.2431