The original buttermilk, from cream collected and left to sit in the cool for a few days until there was enough to churn it all to make butter, was the farm wife's dependable baking powder. Just use it in place of fresh milk, add baking soda sufficient to neutralize it, and voila, fluffy tender pancakes, biscuits, and cakes. (It still gives a better result than baking powder.)
The instructions to "make buttermilk" by adding lemon juice or vinegar to milk are really instructions to produce an acid liquid to replace it in old-fashioned baking recipes calling for buttermilk. The result is not anything you would want to drink, but does supply the acid to combine with baking soda to leaven baked goods. Many people found the original buttermilk left from butter making was a refreshing drink as well as useful in baking, but that was no longer available when the dairy industry turned to using mechanical separators and producing sweet cream butter because that was faster and more profitable.
That's when cultured "buttermilk" appeared in stores as a substitute both for those who liked drinking a cultured dairy milk and for the many who had treasured recipes calling for it as leavening. (The texture and taste of modern commercial buttermilk is, however, a bit different from the original buttermilk from butter making due both to the bacterial culture used and leaving a bit of butterfat in the milk used.) Nowadays, buttermilk is disappearing from stores as many no longer have a taste for it, and most seem to have no idea what to do with it once they've used the 1 cup called for in a single recipe they have listing it as an ingredient. If you don't drink it regularly or make buttermilk biscuits or pancakes several times a week, buttermilk powder is the way to go. It will also store as well as nonfat milk powder in mylar with oxygen absorbers.
Those in the food industry actually have a choice of a few different buttermilk powders of different degrees of tanginess for different purposes. It is all from cultured milk rather than the result of butter making, though. Even Europe is making mostly sweet cream butter now because of the economics of industrialized production. Fortunately, the dairy industry has now created a booming market for whey protein derived from the milk fluid left from both cheese and sweet butter making. They just need to keep those body builders and smoothie makers coming.