I think taking extra vitamins without having a deficiency is a waste of money.
I think, like most things, the response to this is "it depends". It depends on a great many things, but I can really only talk about my own experience.
I was tested for various vits, iron etc and came out as "in normal range" for all of them. However, the range on some of the tests is enormous. And if you are 0.01 within a normal range you will probably be told that everything is fine and there is no need for treatment. Not all gps do this. My aunt has low, but within normal range, vit d levels. She was started on high level supplements by her gp. My elderly mum has virtually identical levels but her gp said no action needed.
I also have hypothyroidism and many people with this function better with vit levels in the higher range of normal. So I have taken the advice of a thyroid charity and upped my levels. My gp said no action needed. But I don't trust my gp.
If you are healthy, eat a good, sufficient diet and get plenty of sunshine you probably dont need any extra supplements. However, if, like many of us on here, you are not healthy (I am not, even tho I see fine if I stop my medication I would grind to a nasty stop), then you might need supplements, even if your gp says "everything is normal range"
So, although most of the population dont need them, many of us do, eve if we don't have an official deficiency.