WAG: So that the makers of counterfeit slabbed coins cannot use the certificate number on their non-counterfeit slabbed coin. 
Thanks for the reply. For everybody's information, they will have to hide the barcode aswell. Especially with NGC if I am not mistaken, since they have a quite nice app called"NGC barcode scanner". I tried it out on some slabs posted here on CFF yesterday and it works scanningon the screen directly. BTW, I am definetly not trying to give tips to counterfeiters by exposing this (they are probably aware)---