To collect as much customer data as possible, to (hopefully) anonymize it and bundle it to sell them to data brokers and extract more money from their customer base...
All of it with their "consent" (you installed the app and likely accepted their ToS/PIvacy Policy), under the pretense of providing them a service like counting the time/frequency of your interaction with the product or reminding you to use it (something you would already do anyways, maybe less accurately, if you are a functioning adult).
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.
All of it with their "consent" (you installed the app and likely accepted their ToS/PIvacy Policy), under the pretense of providing them a service like counting the time/frequency of your interaction with the product or reminding you to use it (something you would already do anyways, maybe less accurately, if you are a functioning adult).