Much as I hate to validate anything the animal id people come up with, I can actually see the point of wanting holding of birth.
I think it is naive to assume that all possible diseases have been identified because everyone has complied with their standstill requirements. Even if all farmers are totally vigilant, some diseases will not evidence until later. Plus, there will be new diseases, for which the epidemiology will be different.
Therefore, if pigs arrive on a holding which then develops a notifiable disease, not only is it useful to know where those pigs came from but also where they originated.
The authorities have that info for cattle, as every single move of every single beast is recorded and held centrally. Increasingly the same is true of sheep (although at present there is no central database so the information would take weeks to collate the first time it's needed.
) And even without sheep EID, the movement forms have for some time required the logging of the flock number in the ears of all the sheep - ie, the holding(s) of birth of all sheep in the consignment.