Why is this piece notched?
As far as I know, it's not intended to have any building function, it's just there to prevent any excess plastic at the fill hole from getting in the way of other pieces.

I can't find a good close-up picture, but that particular piece is filled through a hole in the mold at that location, which you can kinda see in this picture.



Without the small recess, if the sprue were to leave behind a small nub, it could prevent sticking the piece next to another piece.