Call to me, and I will answer you, and will show you great things, and difficult, which you don't know. — Jeremiah 33:3
I tell you, keep asking, and it will be given you. Keep seeking, and you will find. Keep knocking, and it will be opened to you. — Luke 11:9

Author
Charles Spurgeon
Summary: God encourages us to pray because prayer is not merely an exercise that affects our minds: the living God promises to hear and answer His servants. We can call on Him without doubting that He is attentive, trusting that He hears His sons and daughters. When His people cry out in distress, God has wonders and blessings in store for them. His grace can help us in ways we never imagined. All He asks is that we cry out to Him; let us therefore bring Him our prayers with joy and without delay.
God encourages us to pray. People tell us that prayer is a pious exercise that has no influence except on the mind engaged in it. We know that this is not so. Our experience declares that this faithless assertion is a lie a thousand times over.
Here Jehovah, the living God, plainly promises to answer the prayer of His servant. Let us call upon Him once more, and harbor no doubt that He hears and answers us. He who made the ear, shall He not hear? He who gave parents love for their children, will He not listen to the cries of His own sons and daughters?
God will answer His pleading people in their distress. He has wonders in store for them. He will do for them what they have never seen, heard, or dreamed of. He would devise new blessings if necessary. He would plunder sea and land to feed them: He would send every angel from heaven to help them, if their distress required it. He will astonish us with His grace and make us feel that nothing had ever been done quite like this before. All He asks of us is that we cry out to Him. He cannot ask less of us. Let us gladly offer Him our prayers without delay.
Source: The Chequebook of the Bank of Faith. Translation by Allan Roman.
http://www.spurgeon.com.mx/chequera/meditames/junio.pdf