Well, for one thing it contrasted with the human world to show that, despite all the monsters in her fantasy world, the humans in the film were still capable of behaving more savagely. The lack of closure is quite intentional; it's not as if stories like that which occur in the real world have closure either; this verisimilitude is one of the reasons I hold the film in such high estimation. In the end it's not even clear whether the fantastic events were intended to be real or simply a coping device her mind created to escape the brutality of her real life.