What makes this demo import so special?
You get the Demo without the clutter!
Other themes may offer a feature that they call "1-click demo import". But what that really does is clutter your blog with lots and lots of posts, pages and comments. That slows your site down and leaves a mess that you need to clean up manually.
Off the Shelf only installs the exact demo page you want, and you can install other demos later if you like, one by one, just as easily.
Will I lose everything I've already done if I install another demo?
To make it short, no, never!
The demo installation creates a new profile for each demo page so you can keep your previously installed demos. In fact, if you like multiple designs, why not try working with both and see what works best, or develop both versions and test them with your customers?