Horror movies, we are always scared of watching them and you will ever find something that categorizes such movies! Those are media conventions, they make easier to identify the genre in any media production.
It is a convention of the horror genre that side and back lighting is used to create mystery and suspense, we we'll be able to identify it easily, the constant backtracking changes and the way that all movie flows. You are expecting to find blood and a villain, the hero that will be the only one getting out alive, scary monsters and imaginary death puzzles that our protagonist will have to overcome.
Horror films are uncomfortable films designed to make the audience panic, be frightened, it will makes us find our most recondite fears and will use it in order to scare their audience.