Backups are necessary. But, you know, it's nice to avoid restoring a backup if you don't need to.
Furthermore, backups don't help with data that flips bits on disk without being noticed, then consumed by the system in some calculation. A lot of filesystems do not notice non-metadata bits flipping, so you could end up end up with false suddenly becoming true in your database, affecting future calculations.
Once you add RAID, failure conditions become yet more complicated, although some of them become less likely. I don't even know where to begin on that one, except that it isn't a panacea.
It's easy to go "F***" if you're clueless.