See my post #6.
You can steal paper ballots and replace them with other pieces of paper. As long as you can do it in secrecy, no one will be the wiser.
A computer harddrive with a randomly generated digital identity matched with a serial number cannot be easily replaced on a moment's notice. And they are extremely difficult to erase - the only way to hide the evidence is to destroy the original and replace it with an identical physical copy that also has a matching randomly generated difital signature - extremly difficult to do on the fly during a contested election.