Periods are awful to keep track of yourself, it's not a perfect, "every 4 weeks for 5 days" thing. Those apps actually recalculate the beginning and end of a period when something abnormal happens, like stress moving the period back a few days. This way you don't need to keep it in a personal calendar, which mind you, a lot of people don't even have.
Btw, most apps do also more than just regular tracking, they can predict how bad blood flow will be, and if your periods are known to be rather painful, they can keep track and remind you when a day comes on which you'd need to pack a few extra painkillers.