In this particular scene, one could say that Dickens is attempting to humanize the poor. In Dickens's day it was all too common for people to blame the poor for their own sufferings. According to the prevailing prejudice of the time, poverty was primarily due to moral failing; if you were poor, so the argument went, it was your own fault.
This unpleasant attitude is shared by Scrooge, who sees no reason why the poorest members of society shouldn't be sent to prison or to a workhouse if they need something to eat. Scrooge's heartlessness was by no means unusual in those days, as Dickens would've been all too aware. And so Dickens challenges this prejudice, held almost certainly by many of his readers, by showing the poor in a positive light.
That's where the Cratchits come in. They are presented to Scrooge and the reader in stave 3 as a happy, loving family, despite their poverty. The Cratchits may have nothing in material terms, but that doesn't stop them from excitedly preparing for the Christmas holidays. And even though poor Tiny Tim's health continues to deteriorate, he still plays a full part in the family's preparations.
It's clear that Tiny Tim is a loved and treasured member of the Cratchit family, and through him and his family Dickens conveys the important message that the weakest and most vulnerable members of society are human beings, too.