The setting of "The Cask of Amontillado" is important because of its emotional effect on the reader. Montresor lures Fortunato into the gruesome underground catacombs with the promise of a delicious wine. Poe is luring the reader into this same setting by arousing his curiosity about what is going to happen and at the same time making the reader want to see that cask in his imagination and taste it vicariously. But in order to follow Montresor and Fortunato, the reader, in imagination, has to travel through that dark, evil-smelling setting full of human bones. The setting is part of the total experience of the story. It is almost as harrowing as Dante's descent into hell or the earlier descent into hell by Odysseus.
Poe begins the story up on the streets where everybody is making merry. The reader doesn't know what he is in for. He is gradually drawn into Montresor's palazzo, down a flight of stairs into a wine cellar, then along a series of darker passages full of human bones, and finally to the site of Fortunato's immolation. The reader can appreciate the full horror of Fortunato's fate because he can now understand what it would be like to be left down there forever.
Poe also had a plot problem. He wanted to keep Fortunato from talking because his intended victim would surely be asking questions about the Amontillado. Who did you buy it from? How much did you pay? Have you told anyone else about it? Why haven't I heard about the shipment? Why did you store the cask so far from the bottom of the stairs? Where are you taking me? Fortunato could become suspicious if Montresor could not provide satisfactory answers to all the questions he might think of. And Poe has established that Fortunato knows more about Amontillado than does Montresor. Fortunato could ask questions that neither Montresor nor Poe could answer. So Poe provided Fortunato with a bad cold and a cough, making it hard for "his poor friend" to talk. But if there is going to be limited dialogue as the two men wend their way through the catacombs, then Poe will have to fill the space with description, which he does.