Final answer:
In making unshortened and chiffon cakes, sugar should be added gradually to allow proper dissolving and to help stabilize the egg protein structure for a light and fluffy texture. Adding sugar all at once can deflate the batter and make the cake denser.
Step-by-step explanation:
When making unshortened and chiffon cake, you can't add sugar all at once because these types of cakes rely on the careful incorporation of air to create their characteristic light and fluffy texture. Adding sugar gradually while beating the mixture allows the sugar to dissolve properly and helps to stabilize the structure by giving the protein in the eggs more time to unfold and form a stronger network. If sugar is added too quickly, it can deflate the batter by overwhelming the delicate air bubbles and result in a denser cake.