The questions of why we daven, seeing as Hashem knows what we need
and wants to give us whatever is best for us, and why we daven for specific things, despite not knowing if they would actually be what’s best, don’t appear to have anything to do with whether we are using our free will*. Our free will is not about controlling the course of events through prayer (although in a way, it might be possible, depending on the circumstances), but about the moral decisions we make – what we do,
not what Hashem does.
(*Of course, free will must be used in order to daven.)