Fortunato has to be drunk in order for Montresor to be able to deceive and manipulate him. Montresor has a big problem luring this man to his home, down into his wine vaults, and along a winding route through catacombs to the niche with the chains. Montresor wants to keep his victim drunk so that Fortunato won't have his wits about him. Montresor doesn't want Fortunato asking questions about the nonexistent Amontillado, such as where he bought it and how much he paid for it. He doesn't want Fortunato wondering why the pipe of Amontillado has been placed so far away from the bottom of the staircase. It is easy to get a drunken man to drink more. Fortunato remains drunk until he finds himself chained to the granite wall. Then the shock causes him to sober up. Montresor specified at the beginning of his narrative that he wanted his victim to be aware of what was happening to him and who was responsible. When Fortunato cries, "For the love of God, Montresor!" this serves as proof that he knows the identity of the man who is "redressing" the "wrong."
Montresor is the narrator and primary character in Edgar Allan Poe's "The Cask of Amontillado." Montresor feels as if he has been insulted beyond endurance by Fortunato, and he is going to take advantage of the Carnival season to pursue his revenge.
Montresor manages to lure Fortunato to his home, but the burial vault (what Fortunato thinks will be the wine cellar) is damp and Fortunato has a cough. Before they descend the stairs, Montresor offers his guest/victim a glass of Medoc (wine).
"A draught of this Medoc will defend us from the damps."
Here I [Montresor] knocked off the neck of a bottle which I drew from a long row of its fellows that lay upon the mould.
"Drink," I said, presenting him the wine.
He raised it to his lips with a leer. He paused and nodded to me familiarly, while his bells jingled.
"I drink," he said, "to the buried that repose around us."
"And I to your long life."
I can see three reasons for Montresor's offering a drink to Fortunato. The first is the one Montresor suggests, that he does not want Fortunato to succumb to the dampness. This is not a kind gesture, as it seems to Fortunato, but insurance that his victim will not die before Montresor has a chance to kill him.
The second is the opportunity for Montresor to enjoy a satisfying moment of irony. He gleefully makes a toast to Fortunato's long life, knowing full well that Fortunato has only a short time to live.
Finally, it serves as an enticement for Fortunato to continue following Montresor. It is a long way to their destination, and Montresor uses the wine as an incentive for the doomed man to keep walking, despite his cough and the dampness of the vaults.