One of the reasons for period tracking is identifying changes to one's menstrual cycle, which may indicate health issues.[1] So even if periods normally were to only last up to 10 days, not allowing to track for a longer period would impede that goal. In fact, this is even one the vague reasons Fitbit mentioned when it launched this functionality. [2] The functionality feels like a bit of an afterthought.

[1] https://www.medicalnewstoday.com/articles/320758.php

[2] https://www.theverge.com/2018/3/13/17113096/fitbit-watch-per...