Well I do not know anything about Frozen sorbet recipes or egg white as much. BUT... acti white is a replacement for egg white. And I have experimented with acti white in candy. And evety time the acti white was present, the texture I got in the candy was more "spongy" and the appearance of the colour was also more "milky".
So perhaps the egg white is only called for in certain recipes to give a different type of texture that is required for that specific recipe.
Aslo see something I looked up that proves my experience to some degree:
"Eggs are binders which help hold all the ingredients together. Eggs contribute liquid to a recipe and thus serve as a toughener, especially the egg white portion. Egg whites, as mentioned, are often used to produce a light, airy texture, and yolks contribute to the color, flavor, and texture of baked products. But, too many egg whites, such as in a reduced-fat cake recipe make it dry. Including at least one whole egg helps to tenderize. Eggs can also act as leaveners especially when egg whites are beaten separately. The yolk functions to emulsify fat and liquids due to its lecithin content."