Functional application design
creates readable, easily testable application logic,
state of the application is stored easily (seamless / offline experience) and browsed (history / undo),
prevents errors, as usual mutability issues of typical OOP design are greatly reduced.
React/Redux developers usually have in their stack:
ES6 with optional modules and features,
Webpack / Babel with JSX support,
npm, and more recently yarn for packaging.
Also, many React/Redux developers easily fit into React Native development, as the tools are very similar, only the interface is different.
We can also hire a developer to match your framework or preferred solution directly. However, aside from React or Angular, the pool of people to hire from is somewhat shrinking.
You have an existing back-end or API in place and need an awesome user interface for it. You understand the need to have a responsive, fast and usable web part of your customer experience.