The spine’s chilling and the body is shaking

“Nothing is so painful to the human mind as a great and sudden change”
Mary Shelley
 
Horror brings tension to a story, creates a darker atmosphere (intentionally caused by the writer) and awakes the interests inside a reader’s mind, often by expressing questions – who, what, how, why. Questions to which the writer answers with his own style and capabilities. In many crime and drama series the story starts in a calm and peaceful manner before the actual plot starts. The flow of calmness in the story may change a bit over time, I guess. Depends how long the “prologue” part is in a normal, about 40 minute long story. Slowly but surely the story starts to develop and change to a more interesting one, up to a point where the writer wants to drop the first “bomb” of the story on to the reader.
The story named Frankenstein is sort of a benchmark for horror writers nowadays. I would guess that everyone has at least heard the name Frankenstein atleast once in their lifetime. I have here a picture of James Whales’ Frankenstein-story. The picture showcases a supernatural creature, a man-made monster that has been created from stuffing many human (and possibly other creatures’-been decade or two since I read the story) body parts together. The creation called Frankenstein’s monster is walking through an open, sturdy door. Walking after getting new life breathed into it. The picture expresses Frankenstein’s monster to be unnaturally strong and in size much larger in comparison to
a normal human being. A supernatural threat which may or may not be impossible for us to create in the future. Scary.

The use of horror genre in fiction is a great way to make the followers of that particular fictional story see and feel - sense - safely something, that would in real life somebody, like a victim on a crash incident,  traumatized. The human mind tends to want to experience different kinds of colors in their lives every now and then. Young people tend to want to drink alcohol and play slot machines because they're both not allowed for underage people in most countries around the World. The experience of doing something that's illegal makes the experience all the better and makes even something like drinking alcohol exciting.
Horror stories may be based on something that exists between the realms of fantasy and realism. Sometimes people believe seriously in the supernatural and unseen, such as reading stars or tarot cards in order to get a structure to your fate or reading them in order to see to the future, and some people believe in aliens - those things like us from other planets in the Universe. People believe in them despite never seeing them, because it apparently seems ridicilous to believe that we would live alone in the vast space, and Earth shouldn't be the only planet with life on it.
Last notes to this:
What would happen, if aliens were to come and started trying to take over the World?
"There's a wicked witch in the forest, that appears when the sun goes down and kidnaps little children," said an elderly man to his grandchildren in order to not let them go outside during the night.
When you think about it, there are many examples of how people that believe in supernatural all around the World have shown the different ways that this genre has affected their everyday lives, from praying to Greek's gods to learning and studying about Japan's numerous mysterious and mythical creatures.