Just to be clear, native mobile apps are apps that need to be installed via the Play store or the iOS store, while responsive web design are websites can be accessed through the browser and adapt the look and feel depending on the device which is being used to access the website (e.g. mobile phones will display information differently than desktops). But one needs to understand that native mobile apps have much richer functionality than websites. This is because native mobile apps are able to access the native functions of the device on which they are running. Such stuff as built-in notifications, access to the camera, storage, access to information such as contacts, ensures the mobile app has a deeper integration with the device it is installed on. On the other hand, responsive websites only have access to the features which the browser has access to. For security reasons, a browser is sandboxed (or limited in terms of functional access). This is why native mobile apps are necessary and responsive webs design is not enough.