Why did Thomas Aquinas think rewards and punishments would be pointless without free will?

In his Summa theologiae, St. Thomas Aquinas argued that if humans lacked free will, then all "counsels, exhortations, commands, prohibitions, rewards, and punishments would be in vain".1

Aquinas viewed this conclusion as fundamentally inconceivable because the concepts of moral responsibility and justice rely on the assumption that an individual is the author of their own actions. If human actions were not voluntary, there would be no rational basis for holding people accountable through praise, blame, or legal and moral consequences.12

To reconcile this with the belief that God is the prime mover of all things, Aquinas posited that God moves humans in accordance with their voluntary natures. He maintained that just as God moves natural causes without preventing them from acting naturally, He moves voluntary causes without depriving their actions of being voluntary. Thus, for Aquinas, human freedom is not destroyed by divine providence; rather, it is grounded in the fact that God wills each being to act according to its own nature.13