I've been reading various Googled recipes and techniques for stockmaking, as I made my first stock tonight using raw chicken bones. Just about every article/recipe I read says to skim the surface of the stock in the beginning, while it is simmering. Different articles variously refer to the skimmed substances as "scum", "impurities", and "proteins".
I had started out throwing all the vegetables in at the beginning, and probably had it at too high of a simmer, so I never actually got to see any foam or collections of anything other than apparently oils/fats from the chicken appear on the surface. This got me wondering: what is that stuff that floats to the surface? Is there any reason other than aesthetics to remove it from the stock?