Tim Cratchit, best known as Tiny Tim, is a minor but extremely important character in Charles Dickens' A Christmas Carol. He is small and crippled (probably afflicted with either Renal Tubular Acidosis, or Rickets, though Dickens never specified his disease). He is a vital motivator in getting Scrooge to change his ways, and is important to the audience to help us see that Scrooge's change is more altruistic than it would seem without Tiny Tim being there. Without Tim, the audience might think that Scrooge changes his ways simply to avoid his own wretched, lonely end; whereas with Tim in the story, we can see that Scrooge changes his ways because he realizes just how much his selfishness has hurt other people, and is willing to adjust his behavior so as to be able to help Tim and others.
In addition, Tiny Tim's presence helped Dickens drive home a point that was very important to the author; the great disparity in Victorian England between the wealthy and the poor. Without Scrooge's help, Tim was doomed because his family wouldn't be able to afford to treat him, but with the extra income his life is saved.
Dickens seemed to be very sensitive and aware of the societal problems in Victorian England, and wanted to make other people aware of them as well. Using Scrooge, Tiny Tim, Christmas time, and Scrooge's miraculous change, I am certain he helped many people of his age to think a little bit more about the needs of their fellow beings, and offer a little bit more help than they otherwise might have. I'm sure it did, because the story certainly does that, today.
Dickens, Charles. A Christmas Carol. 1844