Bodies don’t have a set schedule for things. An AFAB body is supposed to routinely ovulate and have a period, but lots of things can disrupt that and cause things to happen early or not at all or delayed. Tracking becomes important for a lot of medical reasons, but a common situation in which it’s important to track is for conceiving. And yes privacy is a huge risk with using an app to track, and with how the government in the US is currently talking about punishing women for not conceiving or having miscarriages etc