[ahimel]

In general, most people in developed countries get enough vitamins from their diet. Vitamin deficiency was more of a problem when peasants ate one thing every single day for their entire lives -- if you're an Irish peasant eating nothing but potatoes, you're going to have a problem. In a world where I can trudge through 2 feet of snow to get to a grocery store selling oranges and pineapples, it's not quite such a big deal.

However, depending on the level of junk food in your diet, and the variety you do end up eating, and the sorts of foods you eat, you still may fall short on one or two vitamins. I, for example, can't eat anything very acidic because it hurts my stomach dreadfully - this means I never eat berries and hardly ever eat citrus fruits. As a result, I'm frequently short of vitamin C despite a relatively healthy diet. If you eat neither dairy nor broccoli, you may be low on calcium, and so on.

I take a vitamin pill because it's unlikely to hurt, and fills in whatever gaps there may be. Technically speaking, a better route would be to keep a spreadsheet with all the necessary vitamins and minerals, keep track of how much of each one every item of food has, and make sure I meet the RDA for each one every day. If I fall short of one, I'd take a vitamin pill for that one to make up the difference. But... I'm lazy. So I take a multivitamin, and it keeps my levels for everything high enough that they don't cause problems, and low enough that I don't OD.