Kafka's "The Metamorphosis" is an early example of magical realism and depends just as much on the realism as on the magic for its effect. A pure fairytale could never be so chilling. The point of the story is to imagine a single impossible event, the protagonist suddenly changing into a monstrous insect, and then to describe it as realistically as possible everything that follows from this.
This emphasis on verisimilitude is evident from the first paragraph, as Kafka describes the blanket sliding off Gregor's "brown, arched abdomen" while he lies in bed on his back (an unnatural position for an insect). The details of his bedroom, and his thoughts about his job, are meticulously described, as are the difficulties he now has to cope with following his transformation. Even getting out of bed involves an experimental process of rocking to and fro on his convex back.
The effect of all this realistic description is to heighten the pathos of the callous reaction Gregor receives from his family. He clearly anticipates this, hence his extreme reluctance to open the door, which is fully justified by his father's violent reaction when he does so. There is verisimilitude also in the speed with which Gregor's sister and mother begin to see him only as an insect, with a physical need for space but no emotional attachment to anyone or anything. This may well prompt the reader to think about what would happen, and how those around them would react, if they were to undergo a sudden change, even one much less dramatic than Gregor's.