So few women have it that I question why this thread was even made. And a simple google search would've given OP the answer but whatever...
They do it because it enhances pleasure/sensation due to increased clitoral stimulation. Being a wild, ran though slut and piercings have no correlation outside of the fact that women who are "freakier" and have a lot of sex tend to be more open to finding new ways in which to achieve increased sexual gratification overall. Otherwise, it has nothing to do with just needing to do kegels, being such a thot that no dikk can satisfy them or whatever other bullshyt. Vaginal and clitoral orgasms are different things that originate from different places anyway.
OP, if you just wanted to bash women/"sluts" there were easier ways to go about this outside of asking why 0.01% of the female population decides to do XYZ thing.