Jacob Marley was Scrooge’s business partner in life, and the closest thing he had to a friend. In death, he returns to issue Scrooge a warning, serving as a manifestation of the consequences of greed. He has earned his punishment due to the sins he committed in life and explains that Scrooge can expect the same, unless he changes his ways.
Marley’s presence kickstarts a series of events that leads to Scrooge’s transformation, and it also instills into the story a sense of urgency right from the beginning as Marley explains the depth of his suffering. He is weighed down by the chain of his sins; the links are forged of cashboxes, padlocks, ledgers, and steel purses, things the greedy businessman valued most highly when he was alive and which now constitute a fitting sort of eternal comeuppance. Marley informs Scrooge that he too has fashioned himself a chain, though his is invisible, and consists of an additional seven years’ worth of greed. Part of Marley’s punishment has been to watch Scrooge make the same mistakes he did, unable to warn him, and although Marley is granted no means of escaping his own fate, he capitalizes on the opportunity to warn his only friend.