The process of growing and breastfeeding a child for six months sucks out 5 years worth of nutrition from your body. This means that if you are not supplementing your diet while pregnant and/or breastfeeding your body will remove mineral and other nutrition stores from your body in order to create/ feed that little bundle of joy. Your body will actually take calcium from your bones and give it to your baby. This is one of the reasons women are more likely to suffer from bone loss. (There also another bigger and more complex explanation that has to do with estrogen, but I won't get into that.) So if you plan on having a child in the next five years, or even if you have had a child in the past few years, you need to build (or rebuild) those nutrient stores by taking a vitamin. (This is a tidbit that I didn't learn until I had to start teaching it to people, and is a good reminder for myself because I haven't been good about taking them lately.)
Other good reasons to take a prenatal vitamin: They make your hair and nails grow stronger, faster and healthier. I started taking a prenatal vitamin when I was fifteen for this very reason. (and I took them consistently until I got pregnant, the irony) Also, prenatal vitamins are super cheap. However, you should be forewarned that the iron can make you constipated, so make sure you are drinking plenty of water.