Does prayer make a difference, or would God have done whatever you prayed for (assuming he grants your prayer) anyway?
A little CSL goes a long way. In response to a similar question to the above, he says
“I suppose the solution lies in pointing out that the efficacy of prayer is, at any rate, NO MORE of a problem than the efficacy of ALL human acts. i.e. if you say “It is useless to pray because Providence already knows what is best and will certainly do it,” then why is it not equally useless (and for the same reason) to try to alter the course of events in any way whatever – to ask for the salt or book your seat in a train?”
All this ties in, for Lewis, with the fact that in creating free beings God has “self-restricted” his power in such a way where things actually depend on the action and choices of such agents. To give freedom necessarily implies giving the ability to go this way OR that way, to do good OR bad. And if God made the universe like this (which, since evil exists he apparently did, since he is all good), then evidently he has instituted certain things like prayer which really CAN make a difference in how the whole show unfolds.
That is how he set the whole game up. Evidently, then, how we play it really does matter.