In a sense you’re right; God doesn’t need our prayers. But that doesn’t mean it’s useless to pray, because God has conditioned some events on the agency of his rational beings. If we don’t pray, true, the universe will go on, and it will all be God’s will in the widest sense (that which he causes or permits), but it would not be his will in the stricter sense of what he has enjoined upon his rational creatures, which includes prayer. And the universe will be different because we pray: not because it must be so, but because God made it that way.