While salt and sugar lend flavor, cream of tartar and other acids help stabilize the foam. However, if you add these substances at the beginning of the whipping process, your beaten egg whites will take longer to whip and will never achieve their maximum volume. If you add the extra ingredients near the end, you would not have enough time to incorporate them uniformly into the egg whites before the stiffened mixture starts to become unstable for over whipping. Not only would the foam begin to collapse, it would become increasingly drier because protein molecules would begin to lose their ability to bind with water molecules.
** Asian Recipes