This subject is discussed a lot here.

As near as I can tell, it's just the TDC confirming he's done his job.... And while I do think it's meaningless, earlier this year there seemed to be a point made at certain airports for the paper BP itself to be checked later in the process and the passenger to be admonished if there weren't scribbles, with the assumption being that you must have sneaked into the line and bypassed the TDC.

(Why they can't just assign the checker to watch for line sneakers instead of harassing pax goes unanswered)