Why is Usher's poem included in "The Fall of the House of Usher"?
Edgar Allen Poe includes a song sung by Usher in "The Fall of the House of Usher" as a literary device and as character development and as a plot device. The literary device is the literary technique of foreshadowing that predicts the ultimate sad end of Usher and the literal fall of the House of Usher. Without this device, the story would not have contained a needed clue suggesting its end; all surprise endings must be foreshadowed by clues hinting at the surprise.
The character development shows the seed of the derangement of Usher's mind and reveals to the narrator and, at the same time, to the reader that Usher is losing his mind: "the tottering of his lofty reason upon her throne." This expression equates sanity with "lofty reason," regulating thoughts and perceptions as a monarch on a "throne." Thus to say that there is a "tottering" suggests that sanity is being overwhelmed by the loss of sanity.
The plot device, a literary element, represented by the inclusion of the song/poem is conflict development. The condition of Usher's mind and the root causes are critical to establishing the conflict. Further, the poem sled to a discussion of the sentience of vegetable matter, i.e., plants, that proves critical to the dramatic complicating events following Madelins's death.