1. Poe wanted to increase Montresor's difficulties in consumating his crime by making the victim conspicuous, even having bells on his cap.
2. People choose masquerade costumes to suit their characters. Fortunato likes to make jests like the court jesters of old. Some of these can be cruel. No doubt Fortunato's jests are among the "thousand injuries" Montresor has suffered.
3. Poe describes the costume as "tight-fitting." This would show that Fortunato is unarmed, while Montresor has a sword under his cloak.
4. The tight-fitting costume made it easy to confine Fortunato tightly against the wall with two lengths of chain only two feet apart. This would prevent Fortunato from interfering with the wall-building in that very confined space. If Fortunato had been wearing a cloak or heavier clothing, there might have been a danger of his slipping out of the single chain around his waist.
5. Fortunato is not a fool. Montresordescribes him as "a man to be respected and even feared." The costume characterizes him as a jokester, a man who enjoys playing jokes on people. At the end he pretends to believe Montresor is only playing a joke on him, and he compliments him on "a very clever jest," like a connoisseur of jests. He wants to give Montresor a plausible excuse for changing his mind and releasing him, although he does not not for a moment believe this entrapment is a jest.