Or less cynically¹, people just want to track how well they're brushing their teeth between visits to the dentist. Same reason we track our water intake, calori

Or less cynically¹, people just want to track how well they're brushing their teeth between visits to the dentist. Same reason we track our water intake, calorie intake, heart rate, exercise, menstrual cycles, etc. when we could just use analog tools to do the same thing (or not track it at all). ¹ I'm not suggesting these particular companies aren't selling customer data.