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Watching an episode of Alton Brown's "Good Eats" (episode: House of the Rising Bun) and AB is demonstrating a recipe for cinnamon rolls. In the "wet" mix he includes both buttermilk and butter.
Now, since buttermilk is milk after the butter has been churned out, why would a recipe call for both?
Obviously, proportion would be an issue, but whole milk + (less) butter should equal buttermilk + (full amount) butter, shouldn't it?
Hah, I get to cite my new copy of McGee's "On Food and Cooking" for the first time! There are several things going on here (all of which can be found in the 2004 edition of McGee, most on page 50).
Firstly, as Nathan indicates in his answer, most of the liquid that is sold as buttermilk these days is in fact not "real" buttermilk, but so-called "cultured buttermilk", made from ordinary skim milk and fermented until acid and thick.
Secondly, even traditional buttermilk was somewhat sour, though less than cultured buttermilk. To find out why, let's examine the processes that were involved in making it. If you want to make butter, you start by separating milk into cream and whey. Since the 19th century, we typically do that with a centrifuge and it goes quite quickly, but before then, we would leave it to gravity. That would take a while, and the milk would start to ferment while it happened - especially in warmer environments. This fermenting is essentially lactic acid bacteria converting lactose into lactic acid, which sours the milk a little.
So in either case, you would have more lactic acid in butter + buttermilk than in butter + whole milk.
Two things:
First, commercially available buttermilk is actually milk that has been slightly cultured to increase the amount of lactic acid; that's why it tastes tart. My grandmother used to make 'buttermilk' by leaving some milk on the counter to let it ferment. Because of the increased amount of acid, a lot of recipes (like buns or biscuits) mix buttermilk with baking powder to leaven the mix.
Secondly, the order that they are added might matter. I'm not familiar with the recipe, but he might want to add the butter at an earlier or latter time. He might do the former to cream the butter (mix butter and sugar at a high speed to create bubbles of air in the butter; it's what you do with cake), or he might fold the butter into the mix later on to help create flakiness.
The need to mix buttermilk with butter comes from needing a substitute for sour cream in baking since some people cannot use milk, being lactose intolerant. See "The Milk Sugar Dilemma: Living with Lactose Intolerance", substitutions, page 120, Second Edition, Revised and Expanded by Richard A. Martens, M.D. and Sherlyn Martens, M.S., R.D.
