Dickens uses foreshadowing to hint at the story's forthcoming supernatural events. The narrator's emphasis on death in the first stave signals Marley's imminent return:
Marley was dead: to begin with. There is no doubt whatever about that . [...] You will therefore permit me to repeat, emphatically, that Marley was as dead as a door-nail.
The narrator repeats the fact that Marley is dead "emphatically" and uses the same simile multiple times. He even sounds nervous, as if he is trying to reassure himself that Marley is dead. This creates a sense of foreboding and suggests that everything will not remain as it initially seemed.
With this in mind, Marley's appearance is all the more meaningful because the narrator has taken pains to establish that he's dead—and not just that he's dead, but that he's very dead; as "dead as a door-nail." The emphatic comparison of Marley to a doornail and Scrooge's certainty that he is dead ultimately combine to create a moment of delicious situational irony. when he eventually reappears. Everyone believes that Marley is dead, and the text spends a ridiculous number of words insisting that he will never come back to life, but he does indeed return in the following scene.