Not for the sake of knowing inferior beings does God require messengers, as though He gained aught from them, for He knows all things stedfastly and unchangeably. But he has messengers for oursakes and their own, because to be present with God, and stand before Him so as to consult Him about His subjects, and obey His heavenly commandments, is good for them in the order of their own nature.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: in Gen. ad lit. c. 19.