I can see the appeal in having one ontological world. What is that world, exactly? Is it that which can be proven scientifically (in the sense of, through the scientific method used in natural science)? I think what can be proven scientifically is perhaps what we are most sure is real or true. But things that we are less certain of being real can still exist, as part of the same ontological world. The uncertainty is in us, not in the world. One simplistic definition of natural science is that it is simply rigorous empiricism. The rigor isn’t how we are metaphysically connected with things, rather it’s the empirical that does so, the experiences contacting or occurring to observers. The rigor simply helps us interpret our experiences.
We can have random experiences that don’t add up to anything. But maybe whatever experiences that give rise to our concept “morality”, which we do seem to be able to discuss with some success with other people, and have done so in different time periods, may be rooted in a natural reality (which is not part of the deliverances of “natural science” as “natural” is commonly understood, but which is part of “natural science” if by “natural” we mean “part of the one ontological world”). Morality is something we try hard to make a science of (hence the field of ethics), but which to some extent eludes us. But that doesn’t mean that there isn’t something natural there, but that it’s something we have so far not figured out.
Moral realism can be useful in letting us know what kind of things should be considered moral.
For instance, if you ground morality in God, you might say: Which God? Well, if we know which one, we might know his/her/its preferences, and that inflects our morality. Also, if God partially cashes out to “the foundation of trustworthiness, through love”, then we will approach knowing and obligation themselves (as psychological realities) in a different way (less obsessive? less militant? or, perhaps, less rigorously responsible?).
Sharon Hewitt Rawlette (in The Feeling of Value) grounds her moral realism in “normative qualia”, which for her is something like “the component of pain that feels unacceptable” or its opposite in pleasure), which leads her to hedonic utilitarianism. Not to preference satisfaction or anything else, but specifically to hedonism.
I think both of the above are best grounded in a “naturalism” (a “one-ontological-world-ism” from my other comment), rather than in anything Enochian or Parfitian.