I don't think that was their point. If you went to a bank in the 1940s and made a withdrawal, they wouldn't pull your account slip, erase the balance, and write in the new one. They would add a new line to the ledger noting a new balance. This is by design.
And -- in keeping with the flow of this thread -- a complete luxury until the implements (paper/pens/tape/disk/silicon) would become abundant and ubiquitous.
