Like most data entry software there’s nothing that unstructured notes (or paper) can’t handle.

The main useful feature of the apps (or Apple Health’s tracker which is entirely adequate) is that it sends reminders on the estimated period start date, and then a few days afterwards if you haven’t recorded the end date.

Even “regular” periods often aren’t perfectly regular, or can become irregular when they were regular. (Which is often very important health information.)

It also automatically calculates median period length and typical variation/range.

All unnecessary for some people but very useful for others.