Why did the Ghost of Christmas Present show Scrooge all the poor people helping each other?

What the Ghost actually shows is the Cratchit family celebrating Christmas together. They are the working poor, for, as we know, Bob Cratchit has a steady full-time job as Scrooge's clerk. He is underpaid and gets exactly one paid holiday a year—Christmas Day—but the family is not destitute or living on the streets.
Nevertheless, until he sees them, Scrooge has lacked the imagination to understand exactly how hard it is for the Cratchits to survive on the tiny salary Bob Cratchit earns. (It might cross Scrooge's mind that the man deserves a raise.) The Cratchit home has very little in it, and the family struggles to afford to pull together a very modest Christmas dinner. Scrooge can see that Tiny Tim, the youngest child, is lame and sickly, but also can't help but notice that there is no spare money for him to receive the medical treatments that might save his life.
Scrooge also perceives, however, that the family sticks together and experiences love and joy being with each other as they celebrate the holiday. As he did when he traveled to the past and was reminded of Fezziwig's cheerful and generous Christmas parties, Scrooge realizes that money alone can't buy love and happiness. He may be sitting on a vast pile of money, but Scrooge has little happiness in his sorry, solitary life—in some ways, the Cratchits are better off.
The Ghost wants Scrooge to understand two things: first, that people do need money to survive. Second, however, people are happiest living in loving community.

When the ghost shows Scrooge all of the various groups of working-class people, what Scrooge realizes is that one does not need money to be happy. What one needs to be happy is love: companionship, friends, and fellowship. The miners all celebrate Christmas together and they are happy, as are the lighthouse keepers and the sailors. None of the groups has much, but people come together to share what they have, and this bring them joy—a joy that Scrooge has really never known, at least not since Belle left him. He thinks that he can surround himself with money and be happy, because money will not leave him. It won’t abandon him, like Belle, or die, like his sister Fanny, but it takes him a while to realize that loving and being loved are the only things that make life worth living.