Symbolism, Imagery, Allegory
It's a big deal when Pin becomes a woman in the book, and there's a ceremony and everything. In case you missed it, check out this description of Cam's little sister's warm welcome into womanhood:
Outside, the cold pinched her cheeks with freezing fingers. The fire in the middle of the yard was huge, and so hot that no one stood closer than a handful of paces to it. They all spread out farther when the women gathered on the steps of the shrine. Pin had her bloody cloth in her right hand, the lock of her own black hair in her left. She walked through the crowd and cast both handfuls on the fire. It was done. The villagers whooped and cheered, then jammed around the tables to eat. (14.51)
Pin understandably feels pretty awkward about this, even though she's been around these ceremonies all her life. So why include this ceremony? Couldn't the author have just told us Pin started her period? She could have, sure, but then we wouldn't get the symbolic messages quite as clearly. The ceremony gets our attention, and when it does, we're reminded that the book is set in an ancient setting and that Pin is growing while her brother is away.
So the ceremony doesn't just represent Pin's passage into womanhood—it's also a pretty cool way to mark the passage of time and reminds readers when in time this book is set. We understand that Cam is missing things that matter at home—this ceremony is a major life event, even if it's also majorly mortifying—so when we fast forward through a bunch of years at the end of the book, we remember that things have been happening as time goes by.