What is the added value of using React JS?

Advantages of using React:

• easy to know how a component is rendered, you just look at the render function.
• JSX makes it easy to read the code of your components. It is also really easy to see the layout, or how components are plugged/combined with each other.
• you can render React on the server-side.
• it is easy to test, and you can also integrate some tools like jest.
• it ensures readability and makes maintainability easier.
• you can use React with any framework (Backbone.js, Angular.js) as it is only a view layer.

What is not so good about React?

• it is only a view layer, you have still to plug your code for Ajax requests, events and so on. Some people get surprised by that.
• the library itself is pretty large.
• the learning curve can be steep.

If react-native is really how it was described, react is going to become even bigger.

Performance wise, it is really good as it relies on a virtual-dom to know what is really changing in your UI and will re-render only what has really changed. It does have trouble with very large, slightly changing, lists of children ( <li> test), but can be optimized simply.

Projects using React: instagram, hipchat, facebook chat and so on.

Credit

Simply because React better for performance and memory consumption :)

first of all its so much fun to work with and its light weight  library  it have a lot of features like the virtual dom and so on but what really special about it is that its not really the V in MVC its completely different its divide your web app to component that have all what its need inside it if there is bug its inside the component so when it come to complex project it make it a lot easier and when teams get bigger there is no problem because every member have just component to worry about they don't need to edit something on the project to make there code work