All his reasons are fairly obvious for the technical user and have been discussed in the blogosphere, but I think hearing it from the horse's mouth and at a very high level will benefit non technical users.
I love non-technical users. I was trying to explain to one of them yesterday that they can still use my website even after their school wipes their hard disk, and separately, even though the email address (login) is their home email address and not their school email address. She was quite convinced that I didn't understand the situation, because she was going to lose "all of her memory except the Hotmail at Google", like the tech guy told her.
Here's all the insight I can muster: non-technical people do not have an accurate mental model of how the computer works. They have a semi-accurate mental model of the steps they take to do routine things on the computer: I click this thing, then I type in this thing, then I click this thing, then the magic happens. Deviations from that routine will almost certainly cause task failure.
Example: you know how Macs have the menu bar at the top of the screen rather than at the top of the window? I have at least one customer whose mental model is that the screen ends where stuff stops getting painted to it, and then that grey gunk above it is scary computer stuff. It took a day to talk her through finding the Help menu -- "It says Help, at the top right of your screen" because she was looking under the grey gunk bar and literally did not notice it until I returned the screenshot she had taken with a big red arrow on it.
All the programs and data in your computer is in files and folders, even windows is just a bunch of files. You understand this and everything else is far easier to understand.
My comment was only intended to be about desktop computers.
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