>>12705689he’s saying that they work for people with vitamin deficiencies provided the multivitamin in question has the needed vitamin and no other things that might inhibit the uptake of the needed vitamin.
for example, most people get enough vitamin C in their diet from fruits and vegetables, but if not then taking a vitamin might help prevent scurvy. otherwise, the “use tons of vitamin C!!!” hype is mostly bunk science, since the hype about it boosting your immune system or acting as an antioxidant has not really been borne out and for sure taking a multivitamin with vitamin C daily (or multiple times daily) has not been established to do anything at all in those regards despite lots of research.
another example in multivitamins is vitamin D. most people get enough vitamin D by being in the sun for a few minutes. otherwise, vitamin D milk helps to prevent rickets. some rice is enriched in vitamin D. so most folks are covered. in fact, getting exposure to sun is by far the most efficient way for light-skinned people to meet their quota, since dietary absorption is not nearly as good in bioavailability. now, if you manage to still fail to meet the quota, a supplement would help. however, if you’ve been meeting your quota, then the benefits of additional vitamin D don’t really exist. there was hype about it preventing cancer, but that turned out to be a nothingburger. and in fact, your body basically rejects attempts to increase the supply of vitamin D over what is needed— you tan and reduce your production if you get too much from sunlight, and your body simply stops absorbing excess if you take too much in your diet
