Why do you need buttermilk to marinate chicken before frying?
Rich, slightly acidic buttermilk makes a good marinade for deep fried chicken for a couple of reasons:


The acidity and milk fats help to break down the outer skin of the chicken so it gets crispier as it fries.
It adheres to the chicken and helps the flour coating stick to it.


There is a discussion here that goes through all that and recommends a minimum of two hours marinade time.