If you are trying to tell me you are a PC or Mac user and don't have a back up procedure, then the only thing you are a serious user of is some class 'A' substance imported to you from somewhere hot for a rather large fee. If you have business sensitive or just home files, I really can't believe you don't back up. What and why to back up Files take time to produce. This saves time when problems occur or changing computers. Companies need to take care of customer contacts and databases these days are a valuable tool and require regular backing up to keep a near relative file safe.
Theft is another unknown.
How many people have had their computers stolen either at home or work. It's ok for the insurance to payoff and buy new Macs, but that does not recover the data taken. What to back up on I use a few mediums to protect my data. First I have two online 25Mb storage ftp areas, that I back up anything none sensitive. The second medium is just a super disk for personal data and artwork.