I can give you my reason: I take supplements because I'm positive I'm not getting 100% of my necessary vitamins from my diet/sunlight. Also because on some days I might need more, and on others less, so I make sure that I take enough for even the most "vitaminy-requiring days" (i.e. after infection but before symptoms of a disease, or after workout). That's why I take a dose that's higher than the RDA. There's almost no evidence contrary to taking higher-than-normal doses of vitamins, except for the ones that get stored in fatty acid such as VitA and VitD. For those I watch out that I don't take too much. On the other hand, there's a lot of studies done where people deficient in vitamins have lots of adverse health effects which they may not even be aware of. I figured that there's lots of potential benefits to gain from taking vitamin supplements, and if it doesn't work then I'm only out money.