I believe the clamps might be to hold the box on the back onto the table if you will. With the flap folded over along side of the sorter end of the other boxes, it may have a tendency to push the boxes that are perpendicular to the ones sitting at the "front" of the tables. Especially as they got emptier. Then again, they could also be holding "name tags" on too. Just my guesses based on the picture.
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