I thought it was how they accurately measured the queuing time at security? They were caught out a couple of years back I seem to remember by not telling the full story on queue times (which is one of the measured service quality indicators).
Of course it only works if someone scans your boarding pass at the conveyor, but I've certainly seen that happening recently. They then have the time your boarding pass was scanned at the entrance to the security area, and the time you got to the front of the Q