Because the "A" variant of Code128 allows you to have control characters in there. Many barcode readers act like keyboards to the computer - so the clerk/user positions their cursor to the item number field, waves the item in front of the scanner, and the scanner hardware inserts what it read into the field. Fast, reliable, and easy.

But if the barcode contains control characters, it acts like a keyboard macro and will send them to the application. So if I embed a Ctrl-S in my hostile barcode, I can tell the software to save the record with data I supplied.