The prayer of petition plays an important role in the life of men and women. Although the Lord provides countless blessings without our ever asking for them, He has set aside many graces the granting of which will depend upon our personal prayer, or upon the prayers of others who are close to him. Saint Thomas teaches that our petitions do not change the Divine Will, but instead win for us what has already been set aside for us if we should ask. As a result, we should petition the Lord without ceasing. Who knows how many blessings are waiting for us if only we ask for them. We should ask others to pray for the fervent intentions we have in our hearts, to pray for all the blessings we need and can receive from the Lord. Saint Thomas says that this is one of the reasons why Jesus did not reply immediately to the Canaanite woman. He wanted the disciples to intercede for her. In this way He shows us the importance of the intercession of the saints. The Gentile woman wanted an extraordinary miracle. That required an extraordinary kind of prayer, accompanied by enormous faith and deep humility.
from In Conversation with God by Francis Fernandez Volume Four p.391