For what reasons did medieval writers use allegory?

Read this passage from “the fall of the house of usher.” which element of gothic literature is not obvious in this passage? as if in the superhuman energy of his utterance there had been found the potency of a spell, the huge antique panels to which the speaker pointed threw slowly back, upon the instant, their ponderous and ebony jaws. it was the work of the rushing gust—but then without those doors there did stand the lofty and enshrouded figure of the lady madeline of usher. there was blood upon her white robes, and the evidence of some bitter struggle upon every portion of her emaciated frame. for a moment she remained trembling and reeling to and fro upon the threshold—then, with a low moaning cry, fell heavily inward upon the person of her brother, and in her violent and now final death agonies, bore him to the floor a corpse, and a victim to the terrors he had anticipated. bleak or remote settings supernatural or otherworldly elements macabre or violent incidents strong language full of dangerous meaning