From nakedsecurity.sophos.com
In the early days of personal computers, the main reason people made backups, even if it was just a few important files saved on a special floppy disk, was the sheer unreliability of hardware and software.
If you ever used DOS, you’ll remember very clearly how one buggy program usually crashed everything, and that any crash could leave the hard disk corrupted so badly that you couldn’t reboot at all.
Malware was also a serious concern, not least because the crooks hadn’t yet figured out how to make money out of viruses, but nevertheless often used them to wipe out all your data for no clear reason at all.