The Sfas Emes (Va'era 5643) also brought in the name of the Chiddushei HaRim that through the ten plagues, ten expressions of prayer were opened. All the suffering and bondage that preceded the plagues were a preparation for Israel to come to the level of prayer, for their cries are precious before the Blessed One in heaven (See Likutei Yehuda, Shemos, page 45).