They appear more vigilant to their managers and overseers, for one. If every single traveler is simply ushered through, it could look like they're being lazy (n

They appear more vigilant to their managers and overseers, for one. If every single traveler is simply ushered through, it could look like they're being lazy (not necessarily efficient). And there is a clear bureaucratic incentive to meeting a quota of passengers who will get extra scrutiny. Without marking at least some minimal number of passengers for increased scrutiny, the TSA interrogators, FBI stand-by, etc would have nothing to do and no opportunity to gain real-world "practice" applying their job training and skills.