For starters, it can give you a sense of when your last menstrual began and ended, so you can give your Ob/Gyn or healthcare provider accurate information about the length of your cycle next time you have an appointment. Logging the flow strength and accompanying symptoms can also be a clue into certain health conditions that affect your cycle, such as endometriosis or polycystic ovarian syndrome . And, if you're trying to get pregnant, identifying your fertile window and when you ovulate is crucial.