I'm not an anthropologist, but I think this has been one of the purposes of storytelling in general since the beginning of time. Legends, fairytales, theatre, operas - most of them had scary, violent and tragic themes - not to traumatize or scar people, but to allow them to explore those themes safely, by proxy. I guess storytellers of today (novelists, tv producers, movie directors and the like) are just keeping up with a long tradition - not necessarily for the same purpose, but with the same result.