I thought everyone asked themselves this question at one point and the answer is because it maximizes the manifestation of the good glory of God through our participation and enjoyment of that glory. It is similar to the question, "Why had God sent angels to help him do his will, when he could do it all by himself?". "Why did God ask Adam and Eve to care for the garden when he could have automatically maintained nature himself"? "Why does God ask that we pray, when he already knows our needs and can answer without prayer?", etc. "Why did God make the creation when he was already infinitely satisfied and happy with himself?"
It's a bit wordy and technical but I would like to refer to one's mans careful wording of why God created anything, and then argue from his words an answer to this type of question:
Therefore, to speak strictly according to truth, we may suppose, that
a disposition in God, as an original property of his nature, to an
emanation of his own infinite fullness, was what excited him to create
the world; and so, that the emanation itself was aimed at by him as a
last end of the creation. (Edwards, J. (1974). The works of Jonathan Edwards (Vol. 1, p. 100). Banner of Truth Trust.)
Technically, God must do all things from his own nature because before he created the creation there could be no other motive. God's nature is what defines good, excellent and glorious, notably for his love and grace manifest in the gospel as well as all his other attributes such as power, wisdom, knowledge, omnipresence, etc.
Therefore when God invites us to participate in His will so that we are asked, to ask for it, rather than be mere robotic recipients, we get to enjoy His will as though it were our own and see further into its excellence. We get to share His joy in seeing His nature. The same is true when we pray for others. We engage in God's desire for them and see how holy and blessed His love is. God is generous enough to share Himself with us, even His will to become ours, all for the further manifestation of His glorious attributes to the witness of ourselves, the angels and all of creation. This is represented well symbolically in many passages of revelations where everyone and everything is worshiping the Lamb.