Purity balls themselves are kind of disturbing. Engaging in sexual activity should be a personal choice. It is societies responsibility to teach children the possible outcomes of sexual activity so they can make an informed decisions. Celebrating not engaging in sexual activity makes children think that these activities are bad and should be avoided.
They don't want children to be educated about sexual activity because that usually is correlated with effective family planning, which means fewer children, which means women having more time for personal empowerment rather than simply being vessels for childbirth, which again means fewer children. Fewer children means fewer minds to convert to the religion that motivates the whole exercise. Many 'successful' religions aggressively propagate themselves, even if it's at the expense of the well-being of the believers themselves.
But they need more babies to propogate Christianity;
The popular belief that a celestial Jewish baby who is also his own father, born from a virgin mother, died for three days so that he could ascend to heaven on a cloud and then make you live forever only if you symbolically eat his flesh, drink his blood and telepathically tell him you accept him as your lord & master so he can remove an evil force from your spiritual being that is present in all humanity because an immoral woman made from a man's rib was hoodwinked by a talking reptile possessed by an malicious angel to secretly eat forbidden fruit from a magical tree.
It's half that outcome and half pulling back the rubber band, hoping it stays here when you let go. It doesn't, and instead you have tons of early pregnancy from 18 year olds or younger who didn't understand how to practice safe sex but just knew it was supposed to be great enough to have a ball about not doing until you find the right person.