“Common among their culture” is a quick way of determining whether lots of other people are able to hold this belief and function fine. It also suggests an origin for the belief that isn’t unique to the person. You can’t go around saying “ah this person’s problem is they believe in angels” if the person’s belief in angels is simply part of their religion or culture. “But they believe in imaginary beings” is not a helpful observation in that context.

> “Common among their culture” is a quick way of determining whether lots of other people are able to hold this belief and function fine.

But some people’s functioning is impaired by beliefs common to their culture, whereas other people have very unusual beliefs but aren’t impaired by them to anywhere near the same degree

One person obsesses so much over “culture war” topics they lose their job… another person spends all their free time contemplating elaborate idiosyncratic theories of parallel universes, yet still manages to put a lid on it at work to a sufficient degree to keep their job. The first person has a non-bizarre obsession with serious functional impairment (they lose their job due to it), the second a bizarre obsession but the degree of functional impairment is significantly less. So the fact that we label one obsession “bizarre” and the other “non-bizarre” seems of little relevance