Just my results (n=1) and I don't think this is exactly what you were saying, but just in case other read it the same way I did at first: having had (lab tested) vitamin D deficiencies, vitamin D supplementation can help to restore levels back into the desired range. So supplementation can have the desired effect of improving vitamin D levels (more below). It is a simple test that most doctors don't quibble about adding on to other blood tests (i.e. during annual checkup, for instance), but isn't generally checked by default. (note: insurers may want it to be "diagnostic" rather than "preventative" in order to cover the test.)

Whether it has a "positive impact" on overall health (which I believe to be your point), that would be even more anecdotal and also impossible for me to narrow down whether that one factor had any significant effect, so I won't posit that. And I agree that from different studies I've read, the actual science on it is pretty varied and I haven't seen anything conclusive. Even this study notes their conclusion was "... among adults with suboptimal baseline vitamin D levels".