explain the significance of nelly dean, bringing out clearly her role and function in the novel?

Nelly as the narrator has an advantage no one else in the story has; she has been there through most of the history of Wuthering Heights and has much knowledges that individual characters cannot have. It is safe to say that she has a "vested interest" in the fates of some characters more than others, but she is at least a bit more objective than some of the characters, such as Heathcliff and Cathy, would be.