I think the biggest factor is timing (although backups should also be on a second system). In short, if anything that happens to the files is immediately copie

I think the biggest factor is timing (although backups should also be on a second system). In short, if anything that happens to the files is immediately copied to the "backup" then you don't actually have a chance to recover from any software problems. Whereas, if you make a copy of the data every night and keep the last 30 copies you can find an issue like this and go back in time to retrieve the files from before it started.