As previously stated (numerous times) your BP is checked to make sure you are/are not a selectee and are in the correct lane. The ticket checker should steer you to the correct lane, the WTMD screener ensures you are in the right lane (check on ticked checker). Another screener at the end of the lane (mostly on a dedicated or integrated lane) makes sure you were processed correctly check on ticket checker and WTMD). Some airports who have the ticket checker at the front of the queue have a queue screener at the end to steer you into the right lane.

While this process may seem to be overkill it ensures selectees are properly screened at the checkpoint and the need to screen selectees at the gate or have them come back to the checkpoint can be eliminated thus making it easier all the way around.