To thicken it and to allow the eggs ribbons to form correctly. The cornstarch inhibits some of the protein bonds in the eggs and ensures that they remain silky and not rubbery. Watching the eggs set in beautiful strands is such a fun part of the process. You can choose how thick or thin you would like your broth. This recipes uses 2 tablespoons for 4 cups of broth and it is a light/medium thickness. Simply add more or less cornstarch to suit your tastes.