Isomorphic JavaScript applications are those where JavaScript is used both at the server and client side. Thus, the same React component can be used both at the client as well as at the server side. Some of the advantages of building such applications are:
Whenever required, render the view at the server side based on the application state
The server will render the application in exactly the same way the client would have rendered for increased consistency
In case the JavaScript in the browser is not working, the application would still work because the same JavaScript is present at the server side as well. You need to send the action to the server in order to attain the same result.