Why does Charles Dickens use ghosts in A Christmas Carol?

We know that Ebenezer Scrooge is a man with no friends and also a man who listens to no one but himself.

Oh! But he was a tight-fisted hand at the grindstone, Scrooge! a squeezing, wrenching, grasping, scraping, clutching, covetous old sinner! Hard and sharp as flint, from which no steel had ever struck out generous fire; secret, and self-contained, and solitary as an oyster.

Therefore, Dickens needed someone to communicate with Scrooge in a...

We know that Ebenezer Scrooge is a man with no friends and also a man who listens to no one but himself.

Oh! But he was a tight-fisted hand at the grindstone, Scrooge! a squeezing, wrenching, grasping, scraping, clutching, covetous old sinner! Hard and sharp as flint, from which no steel had ever struck out generous fire; secret, and self-contained, and solitary as an oyster.

Therefore, Dickens needed someone to communicate with Scrooge in a way that would force him to listen, and the ghosts accomplish what no living person can. Fred has already invited him to Christmas dinner and, once again, Scrooge has turned him down. The men at the charity have already tried to get him to empathize with his poorer community members and he told them those people would be better off dead. Bob Cratchit asked for time off for Christmas and Scrooge allowed him half a day if he promised to come in early the next day. All of this tells us he is a man who cares little for anyone but himself. He is truly an isolated and miserable man who has no hope of changing unless something drastic takes place - and ghosts are drastic!

Also, and just as importantly, the ghosts enable to story to take place in the past, present, and future. While all of these scenes might have also been accomplished through dreams rather than ghosts, the dreams would only have been Scrooge's creation and he could have doubted their truth and dismissed them as soon as he awoke. The spirits show him truth in the present and future, which help to genuinely change how Scourge sees his affect on those around him. The journey they take him on is transformative because they have chosen moments in the lives of those around him that Scrooge himself has no awareness of and could not realistically dream about.