The Rule of Metal was a Global Rule. In the creation of the world, it was used as the foundation or the core of the world.
One rule could vastly branch out. Whether the Rule of Earth, the Rule of Water, the Rule of Sky, the Rule of Fire, or even the Rule of Plants, they were all Global Rules branching out from the Rule of Metal.
When Merlin possessed the Rule of Metal, he seemed to know the origin of the world. Metal was the origin of many things.
When he learned the Rule of Metal, it was as if he had studied a good foundation. It was a starting point that could help him learn other rules more easily.
Ever since he became a Demigod, this was the first time he had the feeling that he could actually create a world.
Demigod was the first step towards becoming a God. Even though there was still a long way to go, becoming a Great God who created a world was not beyond his reach.