But there are times when you need at least metaphysical speculation in your world-building. People do have the distressing tendency to think. And they are going to speculate on what exists, and why it exists and whether it has existed forever or not, and what is necessary and what is contingent in the world. And they are likely to be the same sorts of people you consult for help on your quest.
Especially if you have some of the primordial forces of the world running about. If you have forces of Chaos running about, the other side is that of Order. And assuming your heroes are on the side of Order, they will probably have explanations. That Order, perhaps, is primordial and Chaos a revolt against it, as witness that it is always intruding into Order and not vice versa.
How much this filters down into the ground-level religion is another matter. (If your novel is not about the primordial forces, even more so.) But if you are filing off serial numbers and adding your own, it's wise not to knock off parts of the engine. Some religions don't really work without the metaphysics in the background. Buddhism, for instance. Or Christianity.
And if you have an all-wise and all-providing deity, it's wise to relate him to the very structure of the world. Which gets you right into metaphysics right there.
(Yes, actually, I've read some books recently that didn't properly handle the side opposing Chaos, and its god.)