I'd imagine it's the same as personal finance apps. A spreadsheet can be enough, and it is enough for a lot of people, but a custom app can make things easier:

• reducing the friction of keeping track
• built in visualizations
• alerts
• integrating the data with other tools

etc.

I tend to get headaches more frequently at a certain point in my ~monthly cycle, not at the same time I have my period. It's nice to know it's coming so I can plan accordingly. Like avoid being on a road trip at that time, or proactively knocking it out with meds before it even starts.

Some people find their cycles affect their energy and recovery a lot, so they adjust their workout plans accordingly. Like knowing when within a cycle estrogen and testosterone are at their peak, versus progesterone.

Aside from just day count between periods, some people track temp, consistency of vaginal fluid, mood, weight, and probably other things depending on their needs.