The main function of a watch is to tell time. There are other functions. They exist because we want to tell time. The function is related to an agenda... a final purpose.
Democritus said things don't arise to serve a purpose, but having arisen, they find a purpose. Which I think means that if they endure it's because they become caught up in a situation that's reinforcing their existence.
So a naturalist would say people didn't develop thumbs for the purpose of having superior hands. But having developed, thumbs persist because they increased the survival rate of primates that had them.
A theist would say that thumbs are part of a divine plan, as everything is. So they were created in accordance with the agenda of God.
A naturalist would say that a person doesn't have an inherent purpose. Nothing does. Things just arise and fall away. So all purposes are artificial to the naturalist. Therefore good and bad are artificial as well.
The theist would say that a person has a purpose in accordance with the agenda of God, as all things do. Therefore when you think of good and bad, you are seeking to understand God's Will.
If you notice, the two outlooks have some things in common. They both recognize purpose and good as aspects of human life. They differ only in describing the origin of them. The naturalist says that we are the origin. The theist says it's not us, it's God.
So the real difference is us versus other. And this is a pervasive difference between theism and naturalism. In ancient times in the fertile crescent, there was a god of copper smelting. It was understood that smelting was a gift of this god. Today we would find that viewpoint to be ridiculous. Humans created smelting. Apparently the ancients had a problem with owning their own creativity. But we're smarter than that. We understand ourselves to be the latest stage of a progression. In the shadow of this very idea, that we are more advanced, is the ancient perspective peeking out. We never left it behind. We buried it. But it's still there.