I just lost my entire hard drive. I mean, I knew it was going to happen. I dropped the computer (shh, don't tell Apple, not like they care). This happened once before and in 24 hours after it did, I got the white screen of doom. So, once it crashed to the floor this time, I ran and backed up all the important stuff that makes my life work. And it's a good thing, because unlike last time, my hard drive was affected. It's gone. I have a new slate computer here.
The book was backed up. Every day I thank the sweet lord above I did that. And everything truly important - my important documents and movies and pictures and music - all that stuff lives on a backup drive instead of my computer anyway. But there are an uncountable amount of little things that I didn't have backed up, like my giant to do lists on my sticky notes, or some raw footage from my video camera, or my IM conversations from FOREVER. I did a massive backup once, that just copied my entire drive; since then it's been on an as-needed basis, always thinking, "This is just a precaution, I won't lose my hard drive."
YES. YOU. WILL.
BACK. UP. YOUR. STUFF.