Apple uses animated QuickTime elements on a few areas of its site now -- most notably on the iWork page. You can actually scrub through the animation using your scroll-wheel.
I don't know directly how this would be useful; maybe if you missed a certain frame, or if you want to admire all the Apple goodies that flash by without having to watch the movie a dozen times.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060112083243107