Periods can vary in frequency, time, amount of flow, etc. Tracking when it starts and stops, PMS symptoms, and other data points can help to understand if/when something is wrong and give a better upstanding of our cycles. It also helps as we get older and move into the perimenopause and menopause parts of life.

I’ve been tracking my period in a paper planner for a few years now. This tracking helped me link my migraines to the birth control I was on. It has also helped me recognize PMS symptoms to the point where I usually know within a few hours of when my flow will start.

Not everyone gets a period on a consistent basis either. My cycle used to vary drastically from 2-3 times per year to multiple weeks of flow. Menstrual cycles are weird and can be confusing, tracking takes away some of that mystery.

They can vary wildly and it's very useful to keep a track on it

Health, I assume. If the period is irregular, something might be up. And also, when you're fucking, it's good to know if your period isn't late.