In my day job, we see all kinds of data loss due to hard drives failing....and people not having any backups in place. The financial losses for these people would floor you.
My mantra is RAID is for high availability. Backups (on and off site) are used for disaster recovery. Even though these are two completely different functions, both go hand in hand for business continuity.
There were many documented cases after 9/11 where businesses suffered huge financial losses due to data loss...and some never recovered at all. RAID is not much good when your server cannot be recovered. Whereas having remote off site backups and other business continuity plans in place, may have your business back and running in a few hours.
For a recent and very real case of data loss, see this
Google cached page (start reading about 1/2 way down at "Tuesday" then read upwards) of a popular online journal/blog site that went down....all because simple backup procedures were not in place. The site is now back online with different owners and it is a shell of what it used to be...and they probably will never recover from the negative attention the original owners created.
For the average home user, there are a ton of websites that describe in detail all you ever wanted to know about backing up your home computers to any device such as external hard drives, USB drives, SD cards and online backup sites..which I don't recommend.