Awfully painful period cramps are the reason why many girls go on birth control. If you suffer from regular aches and pains when on your period, chances are your body produces high levels of prostaglandin, causing intense contractions and more severe cramping. Since birth control pills prevent ovulation, it in turn reduces the amount of prostaglandin produced in the uterus, alleviating period cramps in the process.

Reference: http://www.mayoclinic.org/healthy-lifestyle/birth-control/in-depth/birth-control-pill/art-20045136/