Having one or two extra bodies around the checkpoint is usually a good thing. Stuff happens that having an extra body around would make easier to handle. A passenger requests/requires a pat-down screening, taking another person away from the "normal" flow of the checkpoint. Another passenger requires assistance in proceeding through the checkpoint (e.g. physical disability). A passenger reports a lost item, requiring a trip back to the office to check the lost-and-found repository. And that's just the "nice" examples.