The android:sharedUserId attribute in the Android manifest is used to specify the shared user ID for an app. The shared user ID is a unique identifier that is used to group together apps that have similar permissions and privileges.

For a system app to be considered a system app, it must have the android:sharedUserId attribute set to android.uid.system. This tells the Android system that the app is trusted and has the same permissions as other system apps.

Here are some of the use cases of using android:sharedUserId for system apps:

1. Communicating with other system apps: System apps often need to communicate with each other in order to perform their tasks. For example, the SystemUI app needs to communicate with the Telephony app in order to display the phone's status bar. By using the android:sharedUserId attribute, system apps can communicate with each other without having to worry about permissions.

2. Accessing hardware resources: System apps often need to access hardware resources, such as the camera or the microphone. By using the android:sharedUserId attribute, system apps can access these resources without having to request special permissions.

3. Performing system-critical tasks: System apps often perform system-critical tasks, such as managing the device's security or updating the system software. By using the android:sharedUserId attribute, these apps can be sure that they have the necessary permissions to perform their tasks.