Or as has happened in a commercial environment(reason for mulktiples) is a file or files can become corrupt but the backup works. Now you have a backup alright...of the corrupted filed. That is why in a business environment there are multiple backup volumes
Backups should always be tested, at least once when the backup scheme is first implemented.
The 3-2-1 Backup Strategy
I use a local drive and two networked drives. A cloud backup service would be much better than a second local network drive. Note that some cloud services (such as Windows OneDrive) remove the local copy so it's not a true backup service.