A simile is a literary device used to compare or contrast two things. When irony is also being used a simile could completely change semantic context and this is why it can be used to compare or to contrast depending on what is being said.
The simile usually uses the prepositions "like" or "as". An example of a simile would be "Your eyes are blue like the ocean.", or "You are cold as ice."
In A Christmas Carol there are several instances of smile that are also great opportunities for the characters to express their emotions, particularly in a story where the main character will undergo dynamic changes. Phrases such as:
I am as giddy as a drunken man
I am as merry as a school boy
I am as happy as an angel
Are good examples of simile where the character gets to explain the extent to which an event has made a mark in his or her life.
The use of "like" is mainly geared toward a physical comparison. In the case of the ghost of Old Marley, the chain that was attached to the ghost reminds the narrator of a tail, sort of insinuating that this apparition in front of him may or may not demon-like. To add depth to the narrative, and to add mystery and horror to the tone, the chain is described just as such
It was long and wound around him like a tail
Certainly to a reader of Dickens's time, this description would have been powerful, as it was during the Victorian period that Gothic literature flourished with the publications of Dracula, Frankenstein, and the likes. Therefore, Dickens used simile to add that "dash" of Gothic imagery that works so well to create the mood of this particular moment in the story.