which families, like his own, were separated as parents were imprisoned for debt. Moreover, Dickens uses the scenes with the Crachits to argue against the prevalent theory of Thomas Robert Malthus in Essay on the Principle of Population (1798), who contended that population growth would supersede food supply, so the poor had no right to live if they could not contribute to the economy and sustain themselves.
Especially moving is Bob Crachit's telling of how Tiny Tim hoped people in the church would see him and recall that Jesus cured the lame and the sick. Even Scrooge is moved by this frail little boy and asks the Spirit if Tiny Tim will live; however, the Spirit informs Scrooge that without the necessary care he needs, Tim will die. He then uses Scrooge's own words about decreasing the surplus population, which echo those of Malthus. When an ashamed Scrooge hangs his head, the Spirit scolds him,

"Will you decide what men shall live, what men shall die? It may be that in the sight of Heaven, you are more worthless and less fit to live than millions like this poor man's child."

Then, after the meal in which all have delighted, they gather around the fire with chestnuts and fill their cups with the "compound from the jug." Bob Crachit makes a toast to Mr. Scrooge and suddenly the delight leaves the faces of everyone. Mrs. Crachit does not want to toast him, but Bob insists "It's Christmas," so she does, although adding some of her thoughts. The children, too, begrudgingly toast their father's employer. But, soon the gloom cast upon them by Scrooge's name is dispelled and the family joyous once again.
As the Spirit whisks Scrooge away, 

...they faded, and looked happier yet in the bright sprinklings of the Spirit's torch at parting, Scrooge had his eye upon them, and especially on Tiny Tim, until the last.

Truly, Scrooge is moved both by the love and happiness in the Crachit family despite their material needs, and by the goodness of all, especially Tiny Tim. whose little frail body does not impair his Christian charity and love for all. 

Many of Dickens's readers will doubtless have shared the noxious prejudices of Ebenezer Scrooge concerning the poor and underprivileged. The general belief in society at that time was that poverty was a moral failing and that the poor had only themselves to blame for their lowly condition in life. In presenting the Cratchit family as poor but respectable, Dickens seeks to challenge this preconception and show that poor people are human beings too.

Though the Cratchits are desperately poor, they don't wallow in self-pity. They accept their lot in life and try to make the best of it. Despite their paltry Christmas feast, they enter into the spirit of the holiday season as enthusiastically as anyone. Contrast this with old "Bah, humbug!" Scrooge, who doesn't celebrate Christmas, despite his enormous wealth.

In giving us and Scrooge privileged access to the Cratchits' home and hearth Dickens aims to humanize the family, showing that wealth, or the lack of it, is no indicator of someone's character. All the Cratchits are fine, decent people who just happen to find themselves at the bottom of the heap through no fault of their own. They are certainly not poor through any moral failing on the part of Bob; he works just as hard as the younger Scrooge did when he had the same job.

Scrooge is so overcome by the warmth and decency of the Cratchits that he asks the Ghost of Christmas Present if poor little Tiny Tim will survive. The Ghost replies that if conditions don't change then there will be no place at the dinner table for Tiny Tim next Christmas. Scrooge is deeply saddened by the news, displaying the same kind of reaction that Dickens would've hoped to have provoked in his readers.

The Cratchit family represent the 'real life' people to whom Scrooge could be kind and charitable, which for Dickens in this novel is a time of giving and generosity more than a Christian religious festival.  Bob Cratchit, Scrooge's clerk, is a poor man with a large family to support. In Stave 1, he is underpaid, and overworked, and bears Scrooge only goodwill, especially at Christmas time. Cratchit uncomplainingly bears with Scrooge's meanness, and is contrasted with Scrooge's nephew, Fred, who is relatively well-off, and only wants to invite his Uncle to a family Christmas party, an invitation which Scrooge rebuffs.

In Stave 3, the Ghost of Christmas Present shows Scrooge images of starving children, and mockingly asks Scrooge 'Are there no prisons? Are there no workhouses?' The Ghost then reveals the reality of the Cratchit's poverty at home, and we understand that Scrooge has no idea until now of the Cratchit family struggles, including the care of their crippled son, 'Tiny Tim'. Still, he is dismissive. The Ghost indicates that the crippled child will be dead by next Christmas.

The Cratchits represent both a moral and political crux for Dickens: Christmas is or should be a time of generosity, materially and emotionally, and in the course of the tale, Scrooge undergoes a moral and emotional transformation and ends by treating the Cratchits to a good Chistmas and - we can infer - saving Tiny Tim's life.

The novel was written (in 1843) at a time when the Poor Laws in England were especially severe - condemning even men with (underpaid) jobs to imprisonment for debt. You should look up the writings of T.S.Malthus on 'The Principle of Population' (1798 - but still a work of note 50 years later) - whose treatise on the ratio of food production to consumers considered those unable to support themselves as virtually unfit to live. It was a well-endorsed political notion at the time, and the workhouses were full of men like Cratchit. Dickens reveals the Cratchits as the human face of these innumerable, dismissable 'poor' and gives them an individual, if highly sentimental, human family life.

If you look at Scrooge as the dynamic protagonist in A Christmas Carol, that is, the main or primary character in a work who undergoes change throughout the story, then the character of Bob Cratchitt (and his family) would be a foil.  He doesn't fit the definition of an antagonist because he isn't working against Ebeneezer Scrooge, but his presence in the novel serves to highlight Scrooge's cruelty and miserly personality. The two are opposites, and the Cratchitts serve to show an example of the virtuous poor.  One of the many themes in A Christmas Carol is that of the perils of wealth vs. the virtuous poor, and it would be impossible to fully explore that without an example of each.  The Cratchitt family exemplifies the virtuous poor - never as clearly as during the Christmas feast when Bob Cratchitt toasts to "the founder of the feast."  Although the rest of his family, especially his feisty wife, is incredulous that Bob wished to bestow blessings on him, they nevertheless did so.  You can't have a Scrooge without contrasting him to a selfless clerk and his son, Tiny Tim.

Dickens uses the Cratchits in two primary ways within the text.  The first is to establish that having money is not a prerequisite for being happy.  While the Cratchits have little food for a Christmas feast, must wear ill-fitting hand-me-down clothing, and must work in demanding jobs, they are still able to maintain their love for each other and their general happiness.  This calls Ebenezer Scrooge's value system into question, as Scrooge values money above all else.

The Cratchits also put a face to the nameless poor.  Earlier in Stave One, when Ebenezer is asked to donate money to the poor, he refuses, saying that he supports only "prisons" and "workhouses."  It is clear from his response that he does not have a clear idea of the plight of London's poor people, and that he is largely unsympathetic.  Showing the Cratchits permits Dickens to highlight what the tribulations of the poor looked like in practical terms.  This, in turn, causes Scrooge to fully understand the situation of poverty at the time, and changes the way he views his role in the situation.

In Stave Three of the story, we meet the Cratchit family as part of Ebenezer's tour with the Ghost of Christmas Present.  While the family of eight (Mom, Dad, and six children) are poor, and while their Christmas "feast" does not feature much food, Dickens states that the family is "still content."  This is so because they have each other, and it is evident from what Scrooge sees that they all have a strong relationship.  While they do not have as much materially as they would like, their relationships, which mean more to them than money, are sound, and they find joy in each other.  This is jarring to Scrooge, who has earlier made statements about the joylessness of the poor.  This also confuses Scrooge because it causes him to question his own value system.  He has all the money in the world, but is not happy, while this poor family wants for material possessions, but manages to be "content" nonetheless.