What is the effect of Kafka's matter-of-fact assertion of the bizarre incidents with which the story begins?
Kafka opens his story as follows:
One morning, when Gregor Samsa woke from troubled dreams, he found himself transformed in his bed into a horrible vermin. He lay on his armour-like back, and if he lifted his head a little he could see his brown belly, slightly domed and divided by arches into stiff sections. The bedding was hardly able to cover it and seemed ready to slide off any moment. His many legs, pitifully thin compared with the size of the rest of him, waved about helplessly as he looked.
Kakfa begins in media res, the middle of the story, as the change into a cockroach has already occurred. This pulls the reader into the action without preamble while the simple language makes it easy to enter into these events. We are not supposed to be concerned with the whys of how he became a cockroach, but to deal, as Gregor must, with the reality of the situation he finds himself in. This is a story not about science but about soul.
Since the story is told at this point from his point of view, his preoccupations become our preoccupations. He is less concerned as he wakes up with being a cockroach than with hating his job as a traveling salesman and with having overslept so badly that he will be late for work. His matter-of-fact response to his change helps us to accept it and suggests that he already considers himself somewhat of a metaphoric insect. It also suggests that his job is so awful that being a cockroach, at least initially, pales in comparison.
It lends a sense of realism that would be difficult to capture otherwise. Without this point of view, many readers would be left fighting to suspend their disbelief, which would detract from the underlying themes of the story. By presenting Gregor's transformation as fact, Kafka is forcing the readers to accept the change, just as Gregor and his family did. It also allows the audience to connect more closely to the events of the story: How would we respond in the same situation?
On the other hand, it also lets us know that we will be entering a world unlike our own, which sets the stage for Kafka's abundant use of irony and black comedy. Even while we are accepting Gregor's new form, we are taking part in a world that doesn't exist, thus we recognize the black humor in the events more readily.