In a sermon titled, “Prayer at Harvest Time: Now!” John Piper said:
Not only did Jesus find a privation among the people, and feel compassion, and see a potential harvest, he also commanded us to pray it in. “The harvest is plentiful, but the workers are few, therefore pray the Lord of the harvest to send out laborers into his harvest” (verse 38). The strangeness of this command points to something very important. It is strange that the farm hands should be told to beg the owner of the farm to send out more workers into his harvest. Surely Jesus doesn’t mean to imply that God doesn’t know there is a shortage of reapers! And surely he doesn’t mean that God doesn’t care whether the harvest comes in! Why, then, are the farm hands told to beg the farmer to get more help?
There is only one possible answer. God has willed that his miraculous work of harvesting be preceded by prayer. He loves to bless the world. But even more, he loves to bless the world in answer to prayer. It is God’s way before he does a great work to pour a Spirit of supplication upon his people so that they plead for the work.