What is so great about Purity Rings?

Why do so many people want them? It just seems like something people use to show off. I'm not a slut or anything, in fact I'm still a virgin, and I don't really see the point. If you want to wait until marriage then isn't that just a promise to yourself? Do you need a piece of metal on your finger to symbolize your chastity? If someone can explain it to me I'll be appreciative. Please, no stupid answers. Thanks.