Vitamins are essential organic molecules that are required for an organism to survive, but cannot be synthesised internally. They are in a separate category from essential minerals, fatty acids, and amino acids. For human beings we require sufficient quantities of vitamins A, B1/2/3/5/6/7/9&12, C, D E and K. Vitamins have diverse biochemical functions and are required for a healthy metabolism to function properly and without them we would develop various diseases such as rickets from vitamin D deficiency or scurvy, without sufficient quantities of vitamin C. The word Vitamine was coined by Polish scientist Casimir Funk in 1912 but the idea that certain foodstuffs contained a substance required to stave off particular diseases was fairly well understood before then. Work by James Lind in the 1740's confirmed that citrus fruits cured and prevented scurvy (vitamin C) and in the 1880's Dr Takaki Kanehiro of the Japanese Navy studied the effects of diet on sailors, realising that an exclusive diet of rice caused beriberi (white rice lacks B1).
For doctors and researchers it was a new idea that the lack of something could cause such horrific and deadly diseases and just a simple diet change could restore health. By the 1950's all the vitamins had been isolated and synthesised and the fad for vitamin supplements took off. The flourishing vitamin supplement industry was firmly backed by “science,” at least in the minds of consumers, and the pharmaceutical industry was happy to provide. As well as vitamins, a whole cartel of supplements were offloaded onto demanding consumers - in 2012 alone they ran up $32 billion in sales in the US alone.
Most skeptics will tell you that if you are 'normal' and eating a varied diet then you do not need vitamin supplements. You are simply creating expensive urine as any excess is simply flushed away by the kidneys. However it is possible to overdose on some vitamins and various national agencies produce recommended tolerant Upper Intake Levels or UILs. Although rare, in 2016, overdose exposure to all formulations of vitamins and multi-vitamin/mineral formulations was reported by 63,931 individuals to the American Association of Poison Control Centers with 72% of these exposures in children under the age of five. Essentially, if you believe you are deficient in some vitamin then you should seek proper medical advice, and if you are taking supplements, it might be a good idea to discuss it with your doctor.