I confess that I do not understand and am frankly skeptical of the phenomenon of purity balls. These are formal social events attended by teenage girls, accompanies by their fathers. They get dressed up in evening wear and tuxedos and dance, kind of like a prom, and the father pledges his efforts to keep his daughter "pure."
I don't have a teenage daughter; I don't have a daughter at all. If I had one, I would certainly want good things for her. I would want her to wait for an appropriate age before having sex. I would want her to not get pregnant, especially while still a teenager. I would want her to be an honest and virtuous person. I would do my best to be a good parent to help make things like that happen. I would do it for her benefit, not mine. I would do those things because that's what a father is supposed to do.
The point of these purity balls, though, seems to be all about the dads, not the daughters. The daughters do not seem to need to make any kind of pledges or promises themselves, although they also seem to be the sort of girls who would take vows of abstinence of their own initiative. Query as to how many of them keep those vows -- I'm willing to be it's less than half, but I don't know where to look to get that sort of information (if it's even reported anywhere in anything approaching a reliable fashion). My guess, though, is that the girls will have sex, or not, in percentages that would be statistically indistinguishable from those who have not taken such pledges, or that would be statistically indistinguishable from those whose fathers have not taken such "purity protection" vows.
So the dad is promising to do something he should be doing anyway, and to extend his authority over his daughter in a way that he realistically cannot control, will not make any difference, and concerning a promise the daughter is quite likely to break. If I weren't so confused, I might cynically say that the whole thing was intended to be an ostentatious public display of moralism, one which I suspect would coincide to a large degree with the sorts of people who engage in ostentatious public displays of piety.
But I also suspect there is a strain of anti-feminism going on here, too; at the least, this is the opposite of feminism. After all, this ritual is intended to make clear to the participants and observers that the father is asserting personal control over his daughter in a very intimate (if chaste) way. The daughter symbolically loses control of her own sexual identity, and must submit to her father's pledges until she is married (and therefore under the control of another man, in this case her husband). This, of course, makes engaging in sex an act of rebellion. So I do not think that this is necessarily the best way to encourage a teenage girl to refrain from sex.
Sheesh. Why don't they just go back to the good old-fashioned chastity belt? (Perhaps because they've become sex toys?)