Apple is tracking us with way more precision than we think.
So now apple does not want you to turn off your Wi-fi. With recent change in iOS 11, the wifi toggle button in control center does not turn off your wifi, rather just disconnect you from wifi. While it doesn't seem much, this little change in feature will allow apple to continuously track you via wifi networks that your phone will detect.
Traditional tracking via GPS/Carrier network can only track you to a certain address (Shopping mall, for example), but tracking via Wifi lets apple track you as precisely as exact location in a building (like, which shop in the shopping mall or which table in the restaurant.)