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.

The following screencast is just one of many tips and tricks from my ebook Back Up Your Mac With Time Machine, which is available for free on the iBooks Store.