I wonder if it’s to prevent “Hackintosh” machines from connecting to Apple services? Services such as iCloud and iMessage require the machine to have a valid serial, and given the format of them is well-known it’s easy to make a valid serial.
Maybe a randomized serial number will allow Apple to sell you a product made in America or Europe without you needing to know the difference. Maybe it also grants Apple greater liberties when negotiating with existing suppliers.
What was the story with Fountain (FC) and Fremont (F) anyway?