Why do some sorbet recipes call for egg whites?
From a food blog:


  I often use egg whites to emulsify (smooth) my sorbets. I have also used lecithin and xanthan gum to great success when trying to not add extra liquid or worrying about someone who is allergic to egg whites.


And another one:


  Others swear by whipped egg white for a creamy consistency


So it seems it makes ice cream creamy. I guess there are other ways to create the same effect. And maybe you don't want to have the same effect across all recipes.