Praying together increases the faith of a congregation to believe God for the miraculous.
Matthew 17 and Mark 9 tell the story of a father who brought his demon-possessed son to the disciples. They could not cast the demon out. The boy was taken to Jesus, who issued a simple command and the demon left. Later, the disciples came to Jesus and they were baffled. Why? Because they knew how to cast out demons. (The story takes place after they had been sent two by two into villages casting out demons and healing the sick.) They asked Jesus why they couldn’t do it.

Jesus said two things: “because you have so little faith” (Matthew 17:20), and “this kind can come out only by prayer” (Mark 9:29).