God is all powerful and all knowing, we are limited in our knowledge and in power so what is the point of prayer? Are we trying to convince God that He should change His mind about how He acts in the world? Are we seeking to inform Him of something that He might not be aware of? Sometimes we do pray as if this were the case, prayer is relation, God has commanded us to pray without seeking. Sometimes God does use our prayers to change situations, not from His perspective but from our ours. Hezekiah would be a case in point, God sent the prophet to tell him to get his affairs in order, but the king prayed and God answered giving him an extra fifteen years. God didn't change His mind about the length of the kings life as the king had not yet fathered the next in line in the Messiah's family. God uses means and prayer is one of those means, that makes us co-workers with Christ, and co-workers with believers around the globe. This same thought is expressed by Bruce Ware:
If anyone thinks that somehow, in a literal sort of sense, our prayers can change God's mind, I would like to ask that person: Who do you think you are?! What could you (or I) possibly know that has escaped God's attention? What perspective do you (or I) have that he lacks? When we consider the extremely limited knowledge we have, our lack of foresight compared to God's perfect foreknowledge, our record of poor decisions and bad judgment in far more cases that we'd like to admit, not to mention our morally twisted natures as as-yet unreformed affections and values, do we really want God to listen to our advice regarding what might be best to do? Honestly, I believe that I could not act more foolishly than to come to God in prayer and suggesting to him that he see things my way and insisting that he do what I want, No, "your will be done" means that another's will precedes mine, and thankfully, this will has been formed our of an omniscient (all knowing) and omnisapient (all wise) mind and heart so that we can be assured it cannot be improved, In prayer,we seek to pray "according to God's will" and in 'Jesus' name, indicating our longing to have our minds, desires affections, and wills reshaped to be more like God's. Bruce Ware Prayer and the Sovereignty of God, in For the Fame of God's Name p.141
Sometimes God uses our prayers to change things and circumstances around us, we are His instruments to bring about His purposes. God always uses prayer relationally to change us.