There are only three reasons to backup: Media failure, inadvertent modification or deletion, and loss in the event of theft or fire. Onsite backup only covers

There are only three reasons to backup: Media failure, inadvertent modification or deletion, and loss in the event of theft or fire. Onsite backup only covers you against the first two. I have found that people tend to overvalue the value of their data to others and undervalue the value of their data to themselves. Having lost all my data in the third category, I can say I'm never going to bother with an onsite backup again; it's simply a false sense of security.