I remember thinking the technique was odd when I first learned it in college. But it has to do with snapping pieces together when building the level. With the origin in the corner, pieces will snap exactly next to each other. If the origin is in the center, the pieces will overlap when snapping.
Though some snapping features in apps/game engines are more robust and can snap via vertices, for example, it’s still a good idea to put the origin in the corner.
