There are examples of structures which only serve a purpose in females, but it seems like the opposite could also be true. Are there any structures which exist in both mammalian sexes and only serve a purpose in males?
Some researchers consider the neural circuitry responsible for orgasm vestigial in women. I disagree with this for a number of reasons, but here is a book exploring the hypothesis:
Females of Rangifer tarandus have horns which only used by males in competitions. Other species of deer only have males with horns.
Apart from this, possibly, clitoris and vulvar lips have no purpose in women: the vulvar lips are underdeveloped halves of scrotum, and clitoris has no need to be so sensitive as it is because it plays no role in sexual reproduction in females.