Why does Hannah feel compelled to tell her friends a story about the future?

Hannah does not tell the story in Chapter 16, but rather in Chapter 18. In Chapter 18, Hannah is watching a few of the girls as they stand pumping water, and while doing so, she suddenly has a vision of two girls in school uniforms playing near a water fountain. Hannah tries to adjust her eyes but the vision remains. She tells the girls that she has a story to tell them and explains that she knows that six million Jews will perish in the Holocaust, but also that the Jewish people will survive and endure. They will have a country of their own and they will even be featured in American movies. The other girls dismiss some of Hannah's claims but she reiterates that they must remember all of this for the future. In doing this, Hannah provides a sense of hope for the future... a reason to go on.