Petitionary prayer, put simply, is a type of prayer where a believer specifically asks ‘God’ to intervene in a situation and change the course of events.
Here is a nice response by a person called Wilstar in the comments section of the article “Governors Urged to Observe National Day of Prayer, Ignore Threats” appearing the Christian Post online, with regards to the apparent futility of petitionary prayer:
Petitionary prayer can’t work anyway and I can prove it. Not by relying on empirical evidence, which also shows that prayer doesn’t work, but by the bible itself. It goes like this: Christians claim that God has a divine plan. The bible also says that the will of God will be done. His divine plan is consistent with his will. So if you pray for something that is already God’s will, the prayer will be answered, but even if you had not prayed for it it would be done if it is the will of God, Regardless of your prayer, God’s will will be done, so why pray? Does prayer change God’s mind? It can’t if God has a divine plan, unless he changes His divine plan, and if he does that for prayer, then his plan is totally capricious. Therefore, prayer doesn’t work by the rules of the bible.
The notion of petitionary prayer does indeed contradict any idea of the omniscience of ‘God’. It is strange that most Christians do not realise this. Their philosophers, however, do recognise that there are serious problems with it:
There is certainly something very strange about the idea of God changing his mind. As God is omniscient, every decision that he makes he makes in light of all of the facts; there cannot arise any new information that God failed to take into account that might cause him to revise his decision. God, then, should never change his mind.
This means that telling God of our needs and asking him to meet them is a waste of time; God is fully aware both of our needs and of our desires, and will have taken them into account in making his original decision. Whatever decision he has made, whether it is in our favour or not, we should not question; our judgement as to what God should do will surely be inferior to his, and so we should let him get on with doing what he is going to do.
It therefore seems that Christians ought not to pray petitionary prayers. Prayers of worship and adoration are understandable, of course, but requests for divine intervention seem to be futile; whatever God is going to do he will do, whatever he is not he will not. Our prayers won’t change that.
So petitionary prayer is a waste of time!
…not that there’s any good reason to think a ‘God’ even exists, in the first place.