A vitamin and mineral supplement will not fix an unhealthy diet on its own, but it can help cover key nutrients if someone is struggling to get them from food. An example of this might be vitamin D where adults in the UK are encouraged to take as a supplement in winter or vegans and vegetarians who might benefit from a supplement of vitamin B12.
Well, it seems they do. Especially for vegans, sedentary people, UK people (no sun at all), etc