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.