It is supposed to make it more tender. Nigella has a roast chicken recipe for a buttermilk marinated chicken - haven't tried this recipe though.http://pickyin.blogspot.com.au/2012/02/ ... icken.html
I have used buttermilk to marinate chicken drumsticks (which I also added grated garlic, paprika and dried mixed herbs to), then patted dry the chicken, coated in seasoned flour, dipped it back into the buttermilk and coated with crushed cornflakes (sprayed it with a bit of oil, then oven baked it at 180 degrees). It was yummo but no more tender than the ones without marinating. That said, I don't think I marinated long enough - esp a cut with bone, I only marinated it for 3 hrs and I think it would have been more tender if I had marinated it overnight. They're good - like a healthier version of Southern fried chicken.