What is the purpose of an egg in a cake batter?

Starr asks: What is the purpose of an egg in a cake batter?

Baking S.O.S. says: Good question!

Eggs have a number of different functions in baked goods and in cakes, in particular:

1) The yolk contains fat, so it gives tenderness to the cake.
2) The white is comprised of some water, so the liquid helps keep a cake moist.
3) The protein in the egg coagulates during the baking process, so eggs help hold everything together, creating structure for the cake (or baked good).
4) When eggs or egg whites are whipped into a foam before folding them into the cake batter (as in a spongecake or angel food cake), the additional air bubbles act as a leavener, helping the cake to rise.  As the air bubbles are heated during baking, they expand, causing the cake to rise.