Without question, my favorite story by Poe. Like so many of his stories, Poe seems to be toying with his audience. Sort of mentally "winking" at us as he pens his words. For Poe (and in The Black Cat), the cat he finds at the bar doesn't simple "follow him home." Instead, the cat, "evinces a disposition to accompany me." Are you kidding me? This is great stuff if read as intended.
It's not until the story has ended and the narrator has taken us along on his fantastic journey that we get the answer to your question. Let me ask you this? What do you think might be the narrator's "immediate purpose" is writing this story?
O.K. now that your done. Think about what it was that the narrator did. What tormented him. What he was driven to do. His punishment? And his atonement? The end of the story has the author about to go somewhere. But before "leaving," he must first clear his conscience. Hence the motivation of this story for the ages.