We are all different, with different diets, lifestyles, nutrient absorption and environmental stress factors.
I have had quite a lot of people say 'just eat a balanced diet', yet when questioned about what that diet should actually be, they always mutter vaguely and are unable to give specifics.
And they certainly can't give specifics tailored to my body, health history and food intolerances.
My doc declined to test for Vit D deficiency, so I paid privately. The result was a clear deficiency, which took months of supplements to rectify (as verified by a second self funded test). I felt MUCH better once my levels were optimum.
All my other blood tests come back normal. Which i am delighted with. However, since i supplement with magnesium and potassium, as well as a few other things (which are definitely not measured with NHS tests), I suspect that if I didn't supplement I would be far too low.

i certainly get cramps and muscle aches/weakness if I forget them for a few days.
I have, for years, eaten a varied diet, 'eating the rainbow', good quality protein, mainly unprocessed, etc. etc. But apparently i need supplements to have normal levels of some nutrients.
C'est la vie.