Protecting your privacy
The more you use your iPhone, the more likely it is that you'll store personal information, such as names and phone numbers in the Contacts app, personal and confidential messages in the Messages and Mail apps, or personal pictures in the Photos app that you may have captured through your iPhone's camera.
While using passcodes, Touch ID, and Face ID can keep unauthorized people from accessing this private data, none of these security measures can keep apps from accessing your private data.
For example, many apps may request permission to access your Contacts data to help you find contact information for your friends. Graphic-oriented apps may also request permission to access your pictures in the Photos app to use them.
If you're concerned that too many apps may have access to your private data, you can selectively block apps from specific data. That way, you can ensure that only the apps that are most important to you have access to...