Buttermilk is a wonderful wonderful baking tool. It adds the right tang to pancakes, breads, batters. The trouble with buttermilk is that it has a short expiration and, at least where I live, you must buy too large of a container to use on short notice.
This is where powdered buttermilk comes in handy. The back of the canister has a handy indicator about how much powder and water to add in to substitute for buttermilk. It keeps in the fridge for ages and relatively speaking, inexpensive and easy to find.