Last weekend MrsDarwin and I took some time out to watch Bend It Like Beckam. (Fun movie.) Anyway, there's a point where the main character's test scores for getting into university have just come, and she's excited to open them, but her mother is insisting on holding the sealed envelope while praying to the family protector (Knowing nothing about Punjabi religion, I'm not clear exactly what he is: ancestor, spirit, god? There's a painting of him in the living room, and I believe they refer to him as "Papa G", though I could have misheard or misremembered.) that the scores will be good.
Now, on the face of it, this seems downright silly, since the test has already been taken and scored, and the scores (whatever they are) are already written down on the paper on the envelope. Even assuming that Papa G has the power to influence the test scores, it's now too late.
Maybe thinking about all this much is a mistake right there, as prayer is (for Christians) at attempt to communicate with a being much different from ourselves. Sacred scripture gives us a number of analogies by which to think about prayer, yet all of them cast the interaction between God and man via prayer in more directly human terms -- as in the parable of the old woman and the dishonest judge.
Taking it that God is both all knowing and outside of time, there is in theory no reason why one should not offer prayers of supplication regarding the outcome of an event which has already occurred but the result of which you are not yet aware of. One would assume that God would have been aware of those prayers before the event occurred. Yet is seems doubly odd to offer prayers after the fact about an event in which you yourself were the primary player. Thus, if you took a multiple choice test, but have not yet received the grades, it seems odd to offer prayers asking God to help you do well on the test, since any last minute inspiration you might have had while taking the test has already taken place. The fact that you don't yet know your score doesn't change the fact that you have already recorded answers which are (by the pre-set criteria of the test) either right or wrong.
It shouldn't come as a surprise that our interactions with the infinite are something that we find difficult to understand. As I said, perhaps one does well not to think on the odder elements of all this too deeply.
Perhaps another thing to recall is that traditional Catholic spirituality teaches that prayers of supplication should be thought of as the least of our prayers. More important are prayers of praise and prayers of thanks. We are, after all, not in nearly as good a position to know what is best for us as God is. The old lady in the parable harangues the dishonest judge constantly demanding that he give her justice. But in her case, she has a better knowledge of justice than the judge does. When we beseech God, the situation is the opposite. Not my will, but Thine be done.