At one level, the difference between online backups and offline backups is that online backups are, well, online; you can make them and get at them without having to load tapes (or hard drives), connect your external USB hard drive, or whatever. There are two advantages of online backups; they involve little or no physical shuffling around of things, and they make for very rapid restores of data.

These advantages should not be understated. It's much easier to automate your backups and make sure that they happen all the time, even weekends and holidays and when you are insanely busy, if they don't require anyone to actually do anything physical. And making restores fast and easy keeps them from draining valuable staff time, especially if the most common restore request is for just a small amount of data.

(With large restores, most of the time can be taken up with writing data back to disk. But with small restores that write only a little bit of data to disk, almost all of the time goes to overhead instead, so reducing the overhead can make a drastic difference.)

However, the crucial difference between online backups and offline ones is that online backups can easily be destroyed, whether by accident or malice. By contrast, destroying offline backups takes actual physical work and is much harder to do by accident (although not impossible). It's thus a good idea to have at least some offline backups, just in case, even if online backups are so much more easy and convenient.