Answer :
The correct answer is that it is a story parallel to what is happening in the Usher house. The poem was actually written prior to the story, but because their plots are so similar, he decided to include the poem in the story afterwards. Both the poem and the story deal with a demented mind, a person who is slowly losing their wits and becoming more and more crazy by the second. So it was only appropriate he should introduce it into his story as well.
It is a story parallel to what is happening in the Usher house.
That is the answer you are looking for