Mans input and use of free choice is so powerful that even the revelation of Hashem depends on the will of man. With this we can understand the mechanics of tefilla. It is not that Hashem changes his mind. Rather all good in the world is contingent on the manifestation of Hashem among us and that depends upon the Jewish people working to reveal Hashem on earth. One way we do this is by davening.
The is also the idea of "בצלם א-להים עשה את האדם" - The revelation of Hashem is brought about by man.
[רזי לי עמ' ע"ד]
