Even people with textbook "consistent" menstrual cycles vary by 3-5 days. People with "inconsistent" cycles vary by weeks or even months. Tracking helps anyone on that spectrum.
I'm towards the more consistent end of the spectrum, but I still use a calendar reminder because my cycle is not a perfect calendar month long so it's not as easy as "my period will start around the 5th of every month". I want to ensure I'm prepared (mentally and with menstruation products) before it starts. Doubly so because I travel for work a fair bit and need to pack for it.
And yes, I moved away from a cloud-based app to a local-only tracker over privacy concerns.
Pretty sure the real answer is Alice; the gays took over the fandom because Kristen Stewart. So Christians and Muslims should defer to ... Wiccans?