Originally Posted by Wyogal
Probably has to do with enzymes or lactic acid acting as a tenderizer.
O.K., where's the science guy?
You're correct. Acids break down connective tissue and tenderize proteins during marinating. This is obvious when you take something highly acidic like red wine and marinate beef overnight. Not so obvious when you use buttermilk to marinate chicken, but the effect is the same. It's not a tremendous effect, but does help retain moisture, give flavor, and tenderize slightly. Regular Milk has very little acidic qualities, only buttermilk does.
Why stop at buttermilk? Try infusing all kinds of flavor into your fried chicken. Perhaps marinate in white wine, garlic cloves, and rosemary sprigs overnight. Now, dredge and fry. Viola! The flavor of Garlic and Rosemary fried chicken without actually having garlic or rosemary IN the item.