In iOS 10 Settings > Privacy > Location, under the "Location Services & Privacy link, it says: "Your iPhone will keep track of places you have recently been, as well as how often and when you visited them, in order to learn places that are significant to you. This data is kept solely on your device and will not be sent to Apple without your consent. It will be used to provide you with personalized services, such as predictive traffic routing." This is consistent with Apple's philosophy of keeping personal data processing on-device as much as possible to protect user privacy. You can always keep the frequent locations feature turned off, or use "Clear History..."
