Sometimes it reveals more of the underpinnings of the world than quite intended.
A character undergoes a strange transformation -- a retroactive, but not perfectly so one -- so that neither she nor anyone else realized it happened, though they notice some odd things.
I threw in a character with the intent that he should briefly sulk over the transformation so as to add to the reader's clues. Whereupon in short order he revealed that he was one of the authors of the problem, having set the thing that caused the transformation.
It's not so much that he has to be revealed in the story, actually, as that he points to sequels where he and his fellows get their handiwork uncovered. . . .