In days gone by, nothing went to waste in the standard homestead, and this included the liquid leftover after churning butter. You might be surprised to learn there is no butter, per se, in buttermilk, and it is lower in fat than sweet milk. It was usually flecked with tiny spots of sweet, creamy butter that didn't quite make it to the top to be skimmed.
Buttermilk soap is especially good for people that have had dry skin, or develop dry skin from the sun or air-conditioning. Skin should never be dried out as it becomes more prone to lines and wrinkles.
Note: These are big bars of Milk Soap (130g) so they last longer. Some people with smaller hands or children may find it easier if the bar is cut into two pieces.