The author includes the prayer that was recited on Hanukkah becuse he/she wants the reader to know that the people living in the annex are not ashamed of their religion and that they are still faithful to Judaism after all they've been through because they are Jewish. Hanukkah is very important to all of the jews living through the Holocaust because it gives them a chance to show that they're not going to give up what they believe in because Hitler thinks it's wrong. Hanukkah makes the people in the annex fell stronger because they're going against the people that have the intent of killing
