Why skim "scum" from the surface of a simmering stock? 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?
Skimming is for aesthetic purposes.

The scum is denatured protein, mostly comprising the same proteins that make up egg whites.  It is harmless and flavorless, but visually unappealing.  Eventually, the foam will break up into microscopic particles and disperse into your stock, leaving it grayish and cloudy.  The more vigorously your stock bubbles, the faster this process will occur.  

If the grayness or cloudiness bothers you but skimming is not an option for some reason, you can always remove the micro-particulates later through the clarification process used to make consomme.