paper calendars work ok. apps are better at collating and predicting based on past data, and therefore giving you a better idea when and what to expect and whether it’s “normal”.

apps can help you provide a condensed report, which helps when seeking help from a doctor. it shouldn’t work that way, but at least in my anecdotal experience, the Dr who dismisses handwritten notes for 3 months, was more reasonable when it was “data collected via app”.

I stopped using an app a few years ago, because of privacy issues, but there are absolutely good reasons people still use them when a calendar works.