Your mail, docs, contacts, and calendar events are actually pretty safe in the cloud. Left on its own, it’s likely that it will remain there, uninterrupted, for the duration of the account’s life cycle. So why would we need a back-up of our mail, documents, etc.?
Two words: user errors.
We’ve all done it before, accidentally deleting an important calendar events, emails or documents. Its not uncommon, and a lot of times, that data can be recovered. But for those instances where recovery is not possible, having a backup isn’t such a bad idea.