Although modern hardware tends to be quite reliable, it can still break seemingly spontaneously. The most critical piece of hardware for storing data is the hard disk, which relies on tiny magnetic fields remaining intact in a world filled with electromagnetic noise. Humans are quite unreliable, they will either make a mistake, or they will be malicious and destroy data on purpose. Nature might not be evil, but it can wreak havoc even when being good. All in all, it is a small miracle that anything works at all.
Backups are a way to protect the investment in data.
It is important to do backups properly. Like everything else that is related to the physical world, backups will fail sooner or later. Or you might notice, when trying to restore, that you forgot to back up something important, like the user database on a 15000 user site. Best of all, all your backups might be working perfectly, but the last known tape drive reading the kind of tapes you used was the one that now has a bucketful of water in it. When it comes to backups, paranoia is in the job description.