MY question is WHY would HE need to do that?
That is how God works.
I (of course) did not say that He NEEDS to do that. He does not need ANYBODY to ever pray ANYTHING for that matter. (He knows everything we pray already.)
However, in His Sovereign, Holy Will, He CHOOSES to have us pray for things, and He CHOOSES to have certain things done through humans (such as writing the books of the Bible).
(You do believe that God chose human beings as instruments to physically write every single word in every single book in the Bible, don't you?)
God could have wrote those things directly Himself, or He could also have just imparted all that knowledge and spiritual wisdom to each of us directly by infusing it all into our hearts, or by some other means, but He CHOSE to make use of human beings to do that. (Do you approve of God's choice to use human beings as go-betweens to write the Bible for the rest of us human beings, even though HE did not really need to do that?)