This is a quite new thing, and I'm sure whatever marketing guys they have at Opera they will explain it better than I can - or better yet, if you install it and try to play with it, you'll see immediately why it's such a great idea. I say 'idea' not a product, because a) it's obviously in a very early beta stage (how do you turn PHP on in the web server?) and b) I'm sure there's many, many limitations that will appear along the way. But it already has plenty of 'wow' factor; maybe there are other applications that enable you to easily host a webserver, mp3 host, photo gallery, chat and message board from your own PC all at the same time (name two?) but Opera gets extra points for style, ease of use and mainstream following.
Combine that with in-browser VNC and Google Gears and your browser really turns into the most powerful tool you will ever need.
Of course, it will probably get hacked into oblivion once it goes live and we'll never hear of it again - until Apple buys it and releases it in black...