Well, they allow it and you to be alive. Without vitamins your organs would not function or exist. They supply rudimentary chemicals that your body needs for everything.

So, take a daily supplement and eat plenty of fruit and vegetables.

If you run low, you get malnourished. And that is NOT pretty. Makes Parkinson's look like a walk in the park.

I spent four months in the hospital due, in large, to malnutrition. I also went blind.
My Neurophthalmologist had me take B vitamins to help with my nerve regeneration.
If you ever get hypocalcemic, or too little calcium, the shot is THE MOST painful shot you could ever get. Avoid it, believe me.