The argument that is put forth in the Prologue is whether Leo has wasted his life. Essentially, has Leo's life been meaningful and worthwhile? The discovery of the diary helps to trigger this argument. It is an argument in which the view of the past is tempered by the present and one in which the view of the present is always influenced by the past. The argument that Leo has between himself as an older man and the younger boy seeks to make a critical judgment on the life that has been lived.
This argument sets the plot in motion. By its nature, the argument seeks to better understand to Leo when he was younger. This means that the novel has to detail what his life was when he was twelve. The plot of the novel is embedded in the argument and its potential resolution. The plot has to examine Leo's life as a young person in that fateful year in order to render a judgment on the argument in some form or another. The debate demands reflection and the plot of the story is what provides this reflection and understanding to both the reader and Leo.