Poe wrote that his main impetus in work of fiction or poetry was to create a mood or feeling. As he put it in his "The Philosophy of Composition,"
I prefer commencing with the consideration of an effect.
In "The Fall of the House of Usher," the effect Poe wants to convey is that of an isolated, dark, sick, and claustrophobic environment and the impact that has on the people living in it. Imagery—description using the five senses of sight, sound, touch, taste, and smell—is vital to conveying this effect.
From the start, the description is relentlessly melancholy, with nothing introduced to lighten the mood. The narrator describes his approach to the house of Usher:
with the first glimpse of the building, a sense of insufferable gloom pervaded my spirit. I say insufferable; for the feeling was unrelieved by any of that half-pleasurable, because poetic, sentiment with which the mind usually receives even the sternest natural images of the desolate or terrible.
In other words, the narrator is indicating from the start that there is no sense of the sublime—the mixture of awe and terror that suggests the grandeur of god—in the scene as he approaches his friend Roderick's home. Everything about it is uniformly and overwhelmingly dismal and bleak.
The dark, somber, joyless imagery conveys extreme nervous tension as the narrator enters the house itself and meets his old friend Roderick, now pale, sickly, and displaying hyper-sensitive frayed nerves and strange behaviors. The house has a "black oaken floor" and
Feeble gleams of encrimsoned light made their way through the trellised panes, and served to render sufficiently distinct the more prominent objects around; the eye, however, struggled in vain to reach the remoter angles of the chamber, or the recesses of the vaulted and fretted ceiling. Dark draperies hung upon the walls....
The darkness is unrelieved except by a lurid red light. Roderick himself plays "wild improvisations" on his guitar and paints strange, "ghastly," and "phantasmagoric " pictures.
All of this imagery conveys the foreboding gloom and perverse strangeness of the narrator's surroundings, setting us up for Madeline's bloody clawing from her tomb.