If it works anything like balloting in the United States, I would guess the one-to-one correspondence is very useful in the case of investigation of actual election fraud - should ballot number A01 be recorded as a Tory vote in a (district? Riding?) over here, and ballot A05 recorded as a LibDem vote over there, by a person that appears to be the same person as A01, spotting and voiding one or both ballots based on the duplication is possible, for example, by auditors or observers tasked with election integrity.