It’s for the purpose of serialization for counterfeit purposes. Also, high end copiers have a device installed called the BDU (Bill Detection Unit) that all scans pass through before being post processed. If the BDU detects a bill being scanned it can error and shut down the whole device until the manufacturer can send someone out to fix it. I used to be one of those people resetting BDUs at schools where a teacher thought it was a good idea to copy images of money for teaching students.