“I work at the post office. I’m told they do this so that the tubes actually get scanned going through the machines (basically a conveyer belt that scans on the top and bottom). Otherwise they spin and/or don’t get scanned correctly.
If they don’t get scanned, the tracking numbers don’t get updated, and people get all mad. By doing it this way, they get scanned at each processing plant.”
—PrometheusAborted
Source: https://www.buzzfeed.com/crystalro/smart-people-reddit