Yes, you need them when you become sexually active for a few very good reasons. If you're not sexually active (any sexual contact AT ALL), wait until 18 or 21, whatever your doc recommends. I've heard both.
1- when you start having sex you are at risk for HPV- HPV can cause cervical cancer, pap tests screen for cervical cancer. You can get HPV and never know, not have any symptoms at all. That's why paps are important.
2- Seeing you gynocologist regularly, you will have the convenient chance to talk about birth control and get prescriptions
3- Once you start having sex, you are at risk for std's- if you aren't practicing safe sex/getting regularly checked, unnoticed std's can develop into pid and cause infertility
4- it's just good to check things out down there and make sure everything's in good working order.