Your iPhone apps can track your location. You already know that, but maybe you tell yourself that that weather app just uses your current location to give you an accurate forecast, or that your bike-routing and tracking app is just keeping a count of miles and calories.
In reality, any one of these apps may be taking that location data and selling it. One way to handle this is to keep up to date with the privacy policies of any location-aware apps you use, but that’s too much work for most of us. Instead, why not just deny them access to your location? On iOS, that’s easy, and it works.