It's a profitable show for the network and relatively cheap to produce. It's an easy way for the entertainment industry to shill their upcoming movies/TV shows/musical acts. Like sports and reality TV it's one of the few products that still lends itself to the network TV model in a world where DVDs and downloads are the place to go for quality fictional programs. You'll see NBC bring back Supertrain before they cancel the Tonight Show. Not going to happen for numerous reasons, including bad blood between Coco and the network and that Lorne Michaels wants Fallon to get it. If anything, Conan might find a way to take over for Letterman someday.