That combination of milk and vinegar is a quick substitution for buttermilk. It provides a different flavor profile from milk alone and adds an acidic ingredient to the recipe that enables baking soda to work for lift.
In addition to what's been said, buttermilk (or sour milk, for that matter) also gives baked goods more of a tangy flavor. Some like that, others don't. I've always been firmly in the "like it" camp.