Eggs are made up of two parts, the egg yolks, and the egg whites. The egg yolks help bind the ingredients together while the egg whites create structure in a cake. They are often called the leavening agents in a cake because they trap air which helps the cake rise. That is why most cakes come with eggs and a specific method to make the most of them in the recipes. For example, whipping eggs separately can make a very light and airy sponge cake such as a genoise or angel cake.