Anyone who uses a computer should subscribe to online backup. Right now, Mozy is both the best and cheapest online backup service out there.
If you create any content on your computer that you would like to keep, you have to back it up. A single hard drive can fail at any time. A single computer can get corrupted, stolen, or destroyed. If you are diligent, you can create periodic backups that allow you restore lost files to some point in the past. But if you are a regular computer user you create so much content — MP3’s, digital photos, Email, etc. — and change it so frequently, that even recent backups will miss things. I had a primary hard drive fail last year, and even though I made a practice of Ghosting it to a backup drive at least once a month, I still lost several weeks of Email and some new work that hadn’t been checked in.
If you are clever, you can rent offsite storage and write scripts to regularly and automatically update critical files. But dealing with open files, versioning, and graceful restores are all non-trivial tasks. Mozy has taken care of all of that, and $55/year is eminently reasonable for unlimited personal backup.