What Difference Does Prayer Make if God Is Already Decided What to Do?

If God is going to do what he wills and he cannot be changed, what difference do our requests make? We all have heard experiences where we either don't get what we'd asked for or we do get the things we never prayed for. I think as people have often said, it's more like a master chess player who can see perfectly well how to win the game. And then the novice on the other side of the board makes a surprising move which changes everything. So beware of these big philosophical categories like immutability because actually when we know the triune God and put Jesus in the middle of that picture and then pray for the spirit to indwell us, things are much more