So here's my thinking—gods are (metaphorically) like gasses, they expand to fill all the available space. And by space, I mean "divine domains". If a potential domain exists, a god will slip into that domain. If multiple gods try to fill the same domain, that leads to conflict.
Now, in the Old World, there is no god of Fatherhood. Morr and Taal are fathers, and Rhya is a goddess of motherhood, but there isn't a specific god of being a father.
And I think Ranald is trying to claim that conceptual space.
Having his own kids, creating a relevant divine artefact, even Mandred—it's all potentially steps in a long plan to establish himself as the God of Fatherhood.
We very nearly saw Ranald take over another unused domain earlier in the Quest—the domain of Magic. That is not held by any human god, and the influx of power from Mork could have helped Ranald claim it. It didn't, in the end—the dice rolled differently—but it's evidence that Ranald can take over new domains for himself. And being the God of Fatherhood is a powerful thing in the Empire.