Why did Edgar Allan Poe believe every word in a short story should contribute to one effect?

Edgar Allan Poe believed that the definitive characteristic of a short story was its "unity of effect." He argued that a skillful artist should first conceive of a unique or single effect to be achieved and then carefully invent and combine incidents that best support that specific design.1

Poe maintained that if a story's initial sentence does not contribute to this pre-established effect, the author has failed in their primary task. Consequently, he believed that every word in the composition should have a direct or indirect tendency toward that single, unified goal. This approach was rooted in his commitment to craftsmanship and artistic integrity, emphasizing that the structure of a story must be meticulously organized to ensure that no element is extraneous to the intended mood or impression.12