Usher reveals that Madeline is his last living relative and only companion. From this detail we know that Usher is extremely isolated and we get a sense of the intensity of his relationship with his sister. Later, he reveals that they share some peculiar connections, and we start to suspect that there is more than DNA in common between these two.
The narrator uses in-depth descriptions of Roderick’s changing appearance to explain the ways he is changing emotionally. Compare the description of Roderick upon the narrator’s arrival to the mansion with the description we get after Madeline’s death, or on the night of Roderick’s own death. Try paying attention to Roderick’s gleaming eye, or the pallor of his skin; the events going down in the house have a huge effect on him, and we get to see that effect physically.
Poe, through his narrator, tells us straight-out that the Usher family is weird, creepy, isolated, old, wealthy, and indistinguishable from their weird, creepy, isolated, old, wealthy mansion. This is important info, and the author doesn’t beat around the bush giving it to us.