I think the author of Learning 2.0 article on del.icio.us site missed the obvious: The best explanation for tagging is smack dab on the del.icio.us website! I read the other cited articles, but they went too deep for me. At least on the del.icio.us site, they gave me plausable reasons as to how and why I would WANT to tag in the first place: planning a vacation, researching a paper, creating a wishlist, etc.
One other thing the del.icio.us site told you was that your tags don't HAVE to be public. You can keep your post private if you want. (I didn't particularly like the one guy's description of "stalking" another user's tags. Sounded creepy to me, even the stalker doesn't know who you are...)
All in all, I can see practical applications for tagging. The best benefit is being able to access your bookmark favorites from any internet access computer.