By allowing him to become nostalgic, the Ghost of Christmas Past was touching into Scrooge's emotions, things he had long discarded. Scrooge as an adult saw the world as a cold and unhappy place. By allowing him to see times when his world was happy, and when he himself was happy, the spirit is preparing him to see Christmas Present with an optimistic and not cynical attitude. If the Ghost of Christmas Present had appeared to Scrooge with no preparation, he would have likely ignored everything he saw.
I’m unsure whether you are referring to Marley's ghost or the Ghost of Christmas Past so I’ll give you an answer for both. Marley’s ghost appears to Scrooge to warn Scrooge that he has been sentenced to walk among the people he avoided in life and to carry the chain that he “forged in life." He warns Scrooge to change the cruelness of his ways, and he warns him that three spirits will visit him over the next three nights.
The Ghost of Christmas Past takes Scrooge on a walk through Christmases past from the time he was a lonely boy to his time as an apprentice with Fezziwig, a kind man, and finally to the time when he loses his love because of his love of power and money. Scrooge begins to realize that this was the point that he began to turn into a bitter man.