Mark 9:25-29 NIV
When Jesus saw that a crowd was running to the scene, he rebuked the impure spirit. “You deaf and mute spirit,” he said, “I command you, come out of him and never enter him again.” The spirit shrieked, convulsed him violently and came out. The boy looked so much like a corpse that many said, “He's dead.” But Jesus took him by the hand and lifted him to his feet, and he stood up. After Jesus had gone indoors, his disciples asked him privately, “Why couldn't we drive it out?” He replied, “This kind can come out only by prayer.”
Jesus didn't pray in the moment he was casting the evil spirit out. He was calling those things which be not as though they were in that moment. Prayer is a separate activity from "calling those things that be not as though they were." Prayer is relational preparation time with God that builds up your faith enabling you to speak God's truth to circumstances ultimately causing those circumstances change and line up with God's truth.
The evil spirit comes out because prayer is the fuel of your faith; prayer is the means by which you deepen your relationship with God!
A lack of functioning in a direct, deep relationship with God can lead to a beating in life!
Acts 19:13-16 NIV
Some Jews who went around driving out evil spirits tried to invoke the name of the Lord Jesus over those who were demon-possessed. They would say, “In the name of the Jesus whom Paul preaches, I command you to come out.” Seven sons of Sceva, a Jewish chief priest, were doing this. One day the evil spirit answered them, “Jesus I know, and Paul I know about, but who are you?” Then the man who had the evil spirit jumped on them and overpowered them all. He gave them such a beating that they ran out of the house naked and bleeding.
You can't go into the battles of life without the preparation of prayer. Prayer helps you to build the spiritual war chest of faith. You speak God's Word out of the wealth of that spiritual war chest. That is the POWER in prayer.