Using multiple backup destinations allows you to use one as your default Time Machine backup disk and others on a less frequent basis. It's a feature that can offers redundancy in the unlikely event something were to happen to both your Mac and its main backup destination.
For example, you could use both a Time Capsule and an external hard drive simultaneously, keeping the hard drive at the office and bringing it home once a week to update. That way, if something were to happen to both your Mac and Time Capsule, you'd still have some form of backup to restore from.
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