I think it's to prepare the mind, in that: if your brain is not trained well enough to isolate the vast amount of fictional stories from our reality, you will have a number of different concepts and ideas prepared in your mind for whatever they introduce next. In that case, both positive and negative work against you, because positives bias towards "it's fine", while negatives can be dismissed as "you're overreacting. It was in that movie.". But in the mean time, in both cases, even if you don't think about it yourself, there is some thoughts available.