It's interesting that you're talking about vit D. Vit D is the one that's the most bullshit of all supplements. Actual vitamin D deficiency causes serious harm and the patient needs to be hospitalized for it. If you weren't hospitalized, it wasn't an actual deficiency.
Just because on average people have, let's say, 100 units of Vit D, it doesn't mean that having 50 units is bad, unless the body reacts badly to 50 units. And with vit D, the body can go very low before there's problems.
Like I was saying in another comment, the optimal values for vitamins or nutrients are more of an average of what a healthy individual has, not necessarily a requirement.
In the same way, having low magnesium is normal if you are a person that sweats a lot or goes a lot to the gym. It doesn't necessarily have to be a pathological problem that requires treatment.
This is a big problem with healthcare nowadays. It doesn't treat the patient, doctors just take tests and make those values look good. It's especially problematic when healthcare is just like any other capitalist enterprise which tries to satisfy the customer, even if that means worse healthcare for them.
The patient comes in with fatigue and instead of asking them about their life and noting that working 60h/week is going to make you tired, they do tests, find vit D and "treat it". I know of many situations exactly like that from people working in healthcare.
We need to stop relying so much on measuring every aspect of our lives with lab tests, step counters, hearth rate sensors, looking at nutrition and calorie intake, etc. We already have a lot of these sensors right in our body and we need to learn to listen to them again.
And no, I'm not some idiot homeopathic pusher either. I take medicine. I take stuff for my allergies, I take some stuff for acid burn, etc. But I do it because I feel that something is wrong (I can't breathe, my stomach hurts) and because it makes me feel better afterwards. A test saying that my vit D is low is not enough to convince me to take supplements.
Now, if you did feel better after taking the supplements, I suppose it's possible that you had a real deficiency that affected your health. In that case it's good you got them. But for the average person to take supplements without feeling bad, that's at best a waste of money. And at worst, it can be worse than not taking them. You know that vit D overdose can cause liver failure, right?
Anyway, sorry for the long text. I won't hold it against you if you don't read it all.