So why would he need prayers to do this? If something is predetermined or it's going to be forced to happen wouldn't it seem to you that prayers wouldn't make any difference?
For the second time again, today, as in
many days prior, the false notion of what God predetermines as occurring 'automatically' shows its face.
The Reformed teaching of the two wills of God —the 'hidden will' of God is called that, as opposed to the 'revealed will' (usually the command), is called 'hidden' for a reason. We often, maybe usually, don't know why he does what he does. But we have his commands, such as to pray. The fact is, we do have revealed in Scripture much about prayer and how he uses it. Note the example in Exodus, where God would have wiped out Israel, had not Moses interceded on their behalf. Yet, obviously he had put Moses there for that very purpose.
The dynamics behind what happens in God's doings goes far beyond what we can comprehend. We can't even begin to write the formula for how a butterfly wing-stroke is instrumental in causing a hurricane on the other side of the earth. God uses
MEANS to accomplish his ends. And he even predetermines those means. He needn't fly by the seat of his pants in mere reaction to whatever by chance comes to pass.
Let's unravel this. Let's imagine God is baking a cake. A lot of things are mixing in the bowl. He's according to you is determining the absolute end result and there will be no question as to how it'll end up. So God is doing A, B, C and D.....but he needs people to come by the bowl to ask him, solicit him to put in ingredients' E, F and G? So why wouldn't he just put in E, F and G regardless. Would he need mere mortal men to make meaningless requests for remember...he's already going to do it? Why couldn't they rightly say, "If you're going to do it anyway why not just leave me alone to play with my kids.....do good for other people.....work some overtime or spend time volunteering. It doesn't seem like God and Jesus to do this. Scripture reveals he's not one who favors waste and doing things that are meaningless.
Your description is faulty. God is also doing the E, F and G. We are involved. This is how he "puts in" the E, F and G. You in framing your own point, say, "It doesn't seem like God and Jesus
to do this." —You are right, HE does this.
eg. When he multiplied the loaves and the fish he told the disciples not to waste any of the left overs. Safe them and don't let anything be wasted! He also states for us to redeem our time....make good use of it for a meaningful purpose. So God has determined everything anyway? So what would our prayers matter anyway?
Your logical jump from his efficiency to 'not needing' (whatever 'need' means, when describing God,) our prayers to accomplish his purposes, is answered in part by what I said above about his hidden will and his command. But also, we know a little about the 'end' he is purposing: the Bride of Christ.
The arguments with atheists are often about this —they want to know, "why does God do things so pedantically, and why must we go through all this —why even bother with sin and salvation and freewill —after all, isn't he omnipotent? Wouldn't it be more efficient to simply speak the end result into existence?". Even my answer to them, (that from his point of view, maybe he did exactly that, having produced through these many thousand years in time, what to him was merely said and done), doesn't answer the question: Why all this? Freewill does not answer the question. Neither does predestination. They may answer how, but not why. Why all this?
But freewill'ers and predestination'ers both know that the production of the Bride of Christ (the Dwelling Place of God, the Church universal, the Body of Christ, the Sons of God etc) is the end, and that 'all this' is what it takes to produce those members of the Bride. We believe in many concepts —walking with God, spiritual growth and maturity, even the individual differences and personalities produced between the members, knowledge of God from our present point of view to be fulfilled when we see him as he is— and so many other things that to us are what he is doing during this temporal, that will not only be completed in Heaven, but overwhelmed (death 'swallowed up' in life) there. Because of
this here, we can,
there, understand in what he has done
here, how great his love for us is.
I say all that to point out that you have the same thing to answer the atheists that you require of Calvinists: If it is all going to happen anyway, why bother with all the details?
Calvinists, at least, have an explanation for what appears to you as inefficiency, which it seems you might be unwilling to use to answer to the atheists: that all this apparent excess of inefficiency is not inefficiency, and (at least) shows what a detailed, specific, beautiful thing and worthy, the Bride of Christ is, because of Christ; this demonstrated by the work and trouble and pain and patience and forgiveness God has gone through on our behalf, for his own glory.