The Home Assistant Companion App provides a convenient way to view and control your Home Assistant instance however it also extends the power of your instance by allowing your device to act as a data source. The Home Assistant Companion App adds numerous sensors (such as battery and network status among others), creates a
device_tracker entity to allow location updates to be sent from the device and also provides action shortcuts to trigger scripts or automations.
Not all features are supported by Android at the moment but eventually most features will be supported. Look for the Android logo to see what is currently supported.