Question about the FLO period tracking app

I’ve seen a lot of news about FLO illegally selling data to meta and google.

I have never used FLO or any period tracker, so i have no clue if it does something special.

So my question is why do a lot of people use it? Can you not track it in your notes app? I understand a lot of women have issues with their periods, me included (pmdd), I just want to ask so i can understand why those apps are needed sometimes :>