From How To Pray by R. A. Torrey

But why is this constant, persistent, sleepless, overcoming prayer so needful?

1. Because there is a Devil - He is cunning, he is mighty, he never rests, he is ever plotting the downfall of the child of God; and if the child of God relaxes in prayer, the devil will succeed in ensnaring him.
2. Prayer is God's appointed way for obtaining things, and the great secret of all lack in our experience, in our life and in our work is neglect of prayer.
3. Those men whom God set forth as a pattern of what He expected Christians to be - The Apostles - regarded prayer as the most important business of their lives.
4. Prayer occupied a very prominent place and played a very important part in the earthly life of our Lord.
5. Praying is the most important part of the present ministry of our risen Lord.
6. Prayer is the means that God has appointed for our receiving mercy, and obtaining grace to help in time of need.
7. Prayer in the name of Jesus Christ is the way Jesus Christ Himself has appointed for His disciples to obtain fullness of joy.
8. Prayer, in every care and anxiety and need of life, with thanksgiving, is the means that God has appointed for obtaining freedom from all anxiety, and the peace of God which passeth all understanding.
9. Prayer is the method that God Himself has appointed for our obtaining the Holy Spirit - If we would only spend more time in prayer, there would be more fullness of the Spirit's power in our work. Many and many a man who once worked unmistakably in the power of the Holy Spirit is now filling the air with empty shoutings, and beating it with his meaningless gesticulations, because he has let prayer be crowded out. We must spend much time on our knees before God, if we are to continue in the power of the Holy Spirit.
10. Prayer is the means that Christ has appointed whereby our hearts shall not become overcharged with surfeiting (overindulgence) and drunkenness and cares of this life, and so the day of Christ's return come upon us suddenly as a snare.
11. Because of what prayer accomplishes.

• Prayer promotes our spiritual growth as almost nothing else, indeed as nothing else but Bible study; and true prayer and true Bible study go hand in hand - As I meet God in prayer and gaze into His face, I am changed into His own image from glory to glory.
• Prayer brings power into our work - If we wish power for any work to which God calls us, be it preaching, teaching, personal work, or the rearing of our children, we can get it by earnest prayer.
• Prayer avails for the conversion of others - By prayer the bitterest enemies of the Gospel have become its most valiant defenders, the greatest scoundrels the truest sons of God, and the vilest women the purest saints.
• Prayer brings blessings to the church - Prayer will root out heresy, allay misunderstanding, sweep away jealousies and animosities, obliterate immoralities, and bring in the full tide of God's reviving grace.