How would we as contingent beings have any certainty about what God as necessary being would or could want, necessarily?
Would necessity entail some sort of determined nature, i.e., impotence, to act as he wills? A coherent concept of omnipotence would preclude that.
Aquinas’ view of God as Actus Purus means that omnipotence entails his will (active principle) is unencumbered. What he wills IS what occurs. He is not a cog in the wheel of necessity constrained by what is or the way things are. He determines what is by the pure act of his will. What he wills is what transpires. His will is “pure act,” the active principle that determines the way things are, not the other way around.
In other words, he is not pure passivity, but, rather, pure activity.