The tease of Valak in Annabelle: Creation is a perfect example of something The Conjuring universe does incredibly well.
The continuity between these films has thus far worked because the directors have handled it with a delicate touch, and the simple act of knowing where Sister Charlotte comes from provides invaluable exposition to help get audiences ready for the next entry in this ever-expanding cinematic world.
Her brief appearance in Sister Charlotte’s photograph (and possibly as the being who pushes Janice’s wheelchair into the barn in Annabelle) not only induces some great screams from members of the audience, but it also helps lay narrative groundwork without weighing down Creation’s core story.