Why do recipes for baked goods that have you combine alternately, at the end, dry ingredients and wet ingredients ...

The theory, as I understand it, has to do with 1) limiting gluten development to ensure a more tender end product, and 2) a better mixture overall. The details as I recall from school: the base should have the fat in it - butter, in your example. When you add dry (usually including flour) to the base, the fat (butter) starts to coat the flour, thus preventing gluten development. You end with dry to absorb moisture in the batter at the end for a better mixture of all the ingredients. We're in Tahoe for the holiday weekend, but I've got a book at home by Shirley Corriher that should provide info on this. I'll look it up next week when we get home and let you know if I see anything additional there.