Using only egg whites in this coconut cake recipe helps achieve a lighter, fluffier texture and a finer crumb. Egg yolks contain fat, which can make the cake denser and richer, while egg whites provide structure and stability without adding heaviness. Additionally, egg whites help maintain the cake's light color, which complements the delicate flavor and appearance of a coconut cake. This is the difference between a classic white cake versus a classic yellow cake.