The Line of Prayer

Paul said in acts 20, verse 23 that the holy spirit testifies to me in every city that afflictions and imprisonments await me. Paul has a great need, and he feels it. He needs help. Help us by prayer. That's the line of prayer. And there are more lessons in this than i can begin to say. But i want to mention two, two lessons from the line of Prayer. Why pray in groups? Why pray publicly? Why not just close the door, like jesus said we should many times, and pray alone?