I always laugh when you hear the phrase "Vitamins have not been proven by the FDA to prevent, treat, or cure any disease". Yeah well nor has broccoli but doctors don't have a problem telling you to eat more of that. The reality there's a reason that they call it practicing medicine. If you have an iron defency they have no problem writing you a script for a pharmaceutical iron supplement, vitamin D issue, they'll write you a script, feeling down or low energy, how about a shot of big pharma made B12 or another script. I'm seeing a pattern here and that that's the big pharma tail wags the dog.
Like anything though it can be exploited for those looking to make money. Is there benefit to saw palmeto, turmeric, or acia berry extract, who freaking knows. Never seen a doctor do a blood workup for a saw palmeto decency though.
I'm not a fan of single multivitamins. I take individual vitamins and try to stay with quality companies. C, D, E, B12, Iron, magnesium, and a fish oil is my daily regement.
Like others have said a balanced diet and adequate water consumption is far more beneficial though. Cut your carb intake down to 30 grams a day and a week later you'll feel like you could run the Boston Marathon.