Why do recipes for beaten egg whites warn you about dryness?

Why do recipes for beaten egg whites always warn you to beat until stiff but not dry and is there a way to keep this from happening?When egg whites are over beaten, they start to lose their moisture, airiness, and smoothness and break down when folded into other ingredients. The miracle solution here is surprisingly easy: use 1/8 teaspoon of cream of tartar for every egg white (1 teaspoon for 8 egg whites). Add it to the whites soon after you begin to beat them, when they start to get frothy. Note: egg white will never beat to stiff peaks if there is it comes into contact with any grease, either from the bowl, beater or even a bit of broken egg yolk.