These inset brick corners can be found all over the older parts of DC. They are especially characteristic of Capitol Hill, where they define bay windows on hundreds of rowhouses. I’m always fascinated by repeating, localized details like that. Why? What was so special about this bit of design that builders decided to use it over and over? Why here and nowhere else?
It isn’t an especially practical detail — it requires several additional steps in the laying of the masonry, and it cuts off the inside corners on the interior. It is a wonderful aesthetic addition, however. It gives the bay window a distinct profile, with a visually slender base and a heavier cap above. This unexpected twist of massing is a common theme of the Queen Anne style and the High Victorian era.
None of that answers the “why”, though. My past research efforts have come up empty, and like much of builder vernacular design, I might never have a more solid answer than the one that applies to many other styles: one builder did it, another one copied it, and it kept on going from there.