Why is it important for Scrooge to experience each one of those Christmases?
Each Christmas that Scrooge experiences reveals something to him. When Scrooge is visited by the Ghost of Christmas Past, he is shown the former joy and trials in his life. Scrooge sees how he lost Belle because of his greed. When he observes the scene of Belle and her husband discussing Scrooge's loneliness, he begs to see no more of the scene. His past Christmases show Scrooge where his greed has brought him.
When Scrooge is visited by the Ghost of Christmas Present, he sees how his greed and selfishness has impacted his present. Scrooge gazes upon the poverty of the Cratchit family, and he hears Mrs. Cratchit's negative opinion of him. He also sees his nephew's Christmas party. He observes the pity and disdain his nephew and niece feel toward him. Scrooge's Christmas present shows him that the people in his life have a poor opinion of him and that they are suffering.
Last, Scrooge is visited by the Ghost of Christmas Yet to Come. He sees that in his death, the only people who show emotion are those in debt to him. He also finds out that Tiny Tim has died. Scrooge at last realizes that he has a chance to change his life and become a more generous, caring person.