It's an analytics feature like those offered by Google and a dozen other analytics SDKs:
https://developers.facebook.com/docs/app-events/
https://developers.google.com/analytics/devguides/collection...
https://developers.facebook.com/docs/app-events/
https://developers.google.com/analytics/devguides/collection...
Who are those people that think it is a good idea to send your medical details (the article also mentions "Flo Period & Ovulation Tracker", which apparently sends whether you may be ovulating to Facebook) to a third-party, let alone Facebook?
(Actually, at least for Android apps there is a answer to who these people are... and there are quite a lot of them: https://reports.exodus-privacy.eu.org/en/trackers/66/)
It's increasingly obvious that iOS and Android need to restrict network connectivity of apps because it's being seriously abused.
Unfortunately this problem is escalating because too few people give a damn about their own privacy, drowning out the voices who do care, deeply, about this issue.
A proper user configurable firewall is the obvious answer, but perhaps also adding limits that an app can only phone home to the domain that the app was signed with.
I feel that both Apple and Google are deliberately perpetuating this problem for financial gain, and should be held accountable as well.
Edit: grammar