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#1390
And what happens if you get a Ransomware attack that encrypts all your devices? Do you keep at least one copy "offline"? What happens if there's a fire at your house? What happens if you delete some data and don't realize it until after your backups are overwritten? OneDrive has file versioning, and can't be affected by Ransomware.
If your data is also local, then there's nothing to worry about. If the MS authentication servers are offline (or your internet is down) you can still log in because credentials are cached locally. Just unplug your computer from the net and try it yourself.