As I understand it if you're not deficient in anything some vitamins are a waste of time and money and others will be helpful. And if you take enough of others it could strain your system filtering them back out, and a few are toxic in large doses.
I take a multivitamin, but I've seen people take an awful lot of pills because 'this one does this and this one does that' and I have to wonder just how much of that is going in and being helpful and how much is just making expensively colourful pee
I have to wonder how many people know why they're taking what they're taking and what it will do, and how it will work with whatever else they're taking, compared to how many people take these things because it seemed like a good idea at the time.
Hmmmm indeed.