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.