I wonder if you would ask the same thing about any number of apps - like fitness trackers, mood trackers, supplement trackers, online diary apps, task trackers, etc? You don't even need a notes app - you could just carry a notebook around or email notes to yourself.

As for why people may want to track menstrual cycles specifically, it is because bodies can be greatly influenced by what phase of the menstrual cycle we are in. From regular physical and mood changes to disorders like PMDD. The different parts of the cycle can also impact ideal exercise and even food choices for some. There are women and couples who gain insights (and often useful predictions) into how their moods coincide with menstrual phases, and that is much easier to track in a dedicated app designed to do so (which can also flag cycle irregularities, bleeding variation, or other changes), just as with other purpose-built applications. All of that is before we even get to the whole fertility tracking thing. One such app is a certified birth control method in my country. Tracking periods in a notes app is not.

> fertility tracking

This. Life is busy and some people just want an app to tell them when they're ovulating.