Although React Native has extensive amounts of native module libraries available, thanks to the huge open source community around it, sometimes there are specific requirements that arise during the course of a project which require native platform API access not provided by React Native. As an example let's say you are working with a client and they have a custom SDK for authentication. The client has SDKs for iOS, Android, and Web. Although you are building a native app for Android/iOS on top of a Web technology (React), the irony is that this task suddenly becomes very difficult.
The beauty of React Native is that it is designed to be a bridge between native code and web technologies. Thus, all you need to do is simply integrate the native iOS and Android SDKs into the respective native projects and expose the methods and variables via a React Native module called:
Thus, if a functionality is technically supported by the native platform, it can be supported in React Native.