It is interesting to note that initially, Scrooge seems to be not impressed at all with the ghost of Marley. He actually goes as far to suggest that Marley's ghost is not real, or that he is just dreaming up its presence. He goes to great lengths to justify why his senses are not to be trusted, and to retain his cynical approach to life and what he is witnessing. Note, for example, how he makes a joke when confronted with Marley's ghost: "There's more of gravy than of grave about you, whatever you are!"
This attitude is completely absent from Scrooge's character when he has endured the three visitations. Note, for example, what he says to the final Ghost who visits him, when he is confronted with his gravestone, and how he demonstrates that his character has reformed:
I will honour Christmas in my heart, and try to keep it all the year. I will live in the Past, the Present, and the Future. The Spirits of all Three shall strive within me. I will not shut out the lessons that they teach.
The ghostly visitations that Scrooge endures, therefore, have the effect of shocking him into realising just how miserly he has become, and they enact the change in his character that he needs in order to restore him to the young, carefree and loving man that he used to be so many years ago. The change in his character is demonstrated in the final chapter when he demonstrates what it means to "honour Christmas" in his heart.
Marley and the three ghosts all contribute a little, and in a different way, in reforming the character of Scrooge.
Marley-Being the former business partner of Scrooge, Marley possessed the same greedy personality and focus on money. Now deceased, he appears before Scrooge as a ghost chained at the waist, brought down by the cashboxes and the keys of his living self. He represents to Scrooge an example of what will happen to him if his obsession over money continues.
The Ghost of Christmas Past-This ghost shows Scrooge a version of himself from long ago, when he was young and happy. Seeing his old self helps Scrooge remember who he was when he was in the company of people rather than money, and perhaps gives him hope that he can return to his past lifestyle.
The Ghost of Christmas Present-Allowing Scrooge to see how others are spending the holiday, it shows his friends and family happy and joyous. In contrast, while Scrooge has a lot of money, he is alone and grumpy. This helps push Scrooge towards the notion that people matter more than money.
The Ghost of Christmas Yet to Come-This ghost impacts Scrooge the most as it shows both Tiny Tim and Scrooge as dead. Tiny Tim died because Cratchit couldn't afford to pay for the boy's medical problems, while no one cares that Scrooge has died. This really affects Scrooge because it shows that while he gained a lot of money, he didn't gain a lot of respect. As well, he could have used his mass wealth to help save a life in Tiny Tim. The Ghost of Christmas Yet to Come, more than any other character, causes Scrooge to reform his ways and to put more focus on being good to others while not caring about money.
Below is the study guide for more information about the story and characters!