UK flooded with forged stamps despite using barcodes — to prevent just that - eviltoast
Royal Mail, the British postal and courier service began switching all snail mail stamps to barcoded stamps last year. The purpose of the barcode was to enhance security, deter stamp reuse, and possibly prevent forgeries—which it has failed to do.

If a stamp have a barcode, why not just let people who have printers at home to print it on the envelope directly? This eliminates the need to buy physical stamp, thus the probability of buying counterfeit stamps.