God knew that He would have a person subject to Him as ultimate king with whom God would have better options than merely hire and fire. Kings like Saul could be anointed, appointed, commanded, but ultimately mainly kept or fired if things went well or badly. God has power but it is coarse grained. A Son is one who God could not only command but also teach and correct, to avoid the nuclear option. The Christ is the main one foretold to David to be this Son who would build the temple rightly. Solomon was initial fulfilment but it established hope of even greater fulfilment in a true Christ. The only-begotten Son. God can therefore be manifestly seen in this Son because the options allow God to show His own ways through this Son by correcting, teaching, helping this Son in all of the ways of God, his heavenly Father.
