It is easy to assume that keeping copies of data on several machines makes it safe, but redundancy and backups protect against different dangers. Redundancy keeps a service running when hardware fails, by having spares ready to take over instantly.
Backups protect against mistakes and corruption: a deletion or a bad change is faithfully copied to every redundant machine at once, so only a separate, older backup can undo it. A system needs both, because surviving a dead disk is no help when the problem is that the data itself was ruined.