What is the purpose of washing pork? I've read in a few recipes, especially ones using gelatinous parts of pork like the head and trotters, that long rinsing under running water, and several changes of water soaking is called for. 

Why is this? 

Why does this practice seem to be unique to pork? 

Thanks
A book I have (Thai Food by David Thompson - a fantastic book, by the way) suggests washing pork and even blanching in boiling water for any dishes involving boiling/stewing/poaching, as it creates a clearer broth with a clearer flavour (author's words, not mine).

I have found I am less likely to get scum and surface impurities in the dish as it cooks.