The new media revolution brought about by the Internet Age leaves a constant vacuum to be filled for the traditional entertainment cycle on broadcast TV. You'll notice a lot of broadcast Web sites doing what they can to fill that void with extra footage, behind-the-scenes stuff, bloopers, "webisodes," and the like.
But let's face it, when the new episodes are exhausted on the networks, we're not likely to stick around for reruns. There's too many other things to do, and we've probably already rewatched the best clips of those shows on YouTube. There goes millions in advertising revenue for the nets.
Basically producers and editors mashed up two old episodes into one hour-long version and threw in never-before-seen footage containing additional sub-plots, etc.
You can read more about it on NBC Universal's March 9 media release.