“What is the value of a prayer meeting?”
Over three years ago now, I wrote a post in answer to the question “Why should I pray if God already knows what will happen?” that dealt with the reasons a Christian ought to make his or her individual prayer life a high priority. If your question is actually a more general inquiry into the value of prayer, you will find several suggestions there which I won’t repeat here.
As to the value of corporate prayer, that’s a separate question well worth considering.