I’m a woman and I have never needed to chart 28 days.

Is this because you don't care when your next period is? Or because you don't need to record it to remember it?

I can imagine a modern woman might not care if she always has menstrual products on hand or nearby. But, it might have been more meaningful in ancient times when there might have been more taboos associated with menstruation, plus it might have been more important to know as part of family planning. And, it might have been much less convenient to carry around whatever was needed to handle menstruation.

Also, in a modern world where calendars are everywhere, I can imagine someone might say "ok, so my next period will be in early July". But, there was a time when days and months were not tracked, or were only tracked by priests, etc. In that kind of situation, I could imagine it might be useful to count the days until the next period was expected. On the other hand, a primitive society probably spends a lot more time outdoors and sees the moon a lot more often, so it might be just as easy to go "ok, so my next period will be when the moon's 3/4 full".

28 notches means that the bone had 29 sections, which more closely matches a lunar month than a typical menstrual period. But, I could see it being used either way.