Click here for mania.com tie-in comic books interview with Keith R.A. DeCandido, the author of Supernatural tie-in novels Nevermore, Bone Key and Heart of the Dragon.
"In the case particularly of comics like Buffy, Angel, Farscape and the new or planned post-finale comics for Jericho and Pushing Daisies, it's a way to give the hardcore fanbase more stories. If a property's audience has dropped to the point where it can't support a TV show (which requires an audience of millions and millions to be successful), but it has a devoted following that can support a comic book (which only sells in the thousands), then everyone's happy."
DeCandido was also interviewed by The SciFi Diner Podcast (you can listen to the pocast here), where he talked about Supernatural and gave an insight on Heart and the Dragon and the upcoming The Unholy Cause (by Joe Schreiber) and War of the Sons (by Rebecca Dessertine and David Reed).
Source: Keith R.A. DeCandido LiveJournal.