why do you think the narrator provided a flash back to hans hubermann's World War 1 experience

Part Four provides this flashback in order to help us better understand his character. Part Four of the novel solves the question of whether or not Hans is a coward. We learn that Hans' life is miraculously saved by a Jew in World War I, and for the rest of his life Hans is gracious to Jews despite the threat of imprisonment in a concentration camp for helping them in the wake of Kristallnacht -- indeed, Hans is lucky that he is not taken away for vandalizing a Nazi Party office. Risking his life to hide Max is Hans' courageous payback for Max's father Erik's good deed. Hans feels guilty over the fact that Erik, who had a son, died while childless Hans survived. By caring for Max, who still suffers from the loss of his father at a young age, Hans performs an important fatherly duty.