The need to keep back-up copies of software arose when programs and data were no longer stored on paper media, but were entered from terminals and stored on disk.
Tape archives are suitable for long-term, offline storage.
If all changed files are dumped on a back-up tape once per day, old revisions remain accessible. However, tape archives are unsatisfactory for version control in several ways. First, backing up the file system every 24 hours does not capture intermediate revisions. Secondly, the old revisions are not online, and accessing them is tedious and time-consuming. In particular, it is impractical to compare several old revisions of a group, because that may require mounting and searching several tapes.