Here's an excerpt from my article on Medium:
Rails developers should learn React because there's growing market demand for developers who know both. Learning to use React will make you more money and a better developer.
In many ways, React is to the modern frontend, what Rails is to the backend - there are many alternatives, but React has the best combination of simplicity, power and community momentum to make it a great choice.
Web UIs have matured a lot over the last decade. We're doing more complex things to build better user experiences on the web. jQuery has served well but even a medium-sized project can now quickly turn into the Flying Spaghetti Monster. React offers a simple yet powerful alternative to spaghetti code.