To be a structural device in the plot of the novella: the story opens with his death, then moves forward exactly 7 years to his reappearance as a ghost, setting the supernatural tone
To show that Scrooge had only one friend in his later life
To act as a warning for society of the consequences of a life of greed and selfishness.
To demonstrate that selfishness and greed will harm oneself. Marley is trapped in an afterlife of eternal remorse and regret
To offer the hope of insight, change and redemption to Scrooge