In the beginning of Parshas Toldos, we learn that Yitzchak and Rivka have been married for 20 years but have no children. The reason is because Rivka is barren. Rivka and Yitzchak both begin praying and suddenly Rivka has children: Eisav and Yaakov. From Yaakov, who is also called Yisroel, comes the 12 tribes, who are the fathers of all of the Jews. If Yitzchak and Rivka did not pray, Rivka would have never given birth to Yaakov and the Jewish people would not exist. We only exist because of prayer.