AngularJS is a full-featured front-end framework. It takes care of the three parts of MVC, plus the structure of the overall application (dependency injection, routing, ajax, etc.).
You don't need React for Angular. You actually don't need anything else than Angular.
ReactJS is a specific library for the "V" part of MVC. It's only about the view. If you don't need anything else, just use React. If you need routing, ajax, events handling, etc., the philosophy of the React/Flux stack is to use specialized libraries for that (react-router, some Flux implementation, Immutable.js, etc.).
These are two different approaches. You can replace Angular's directives with React's components, thus using Angular for the "MC" and React for the "V", and there are some benefits of this (mostly performance, also, React components are simpler to reason about than Angular directives).
If you are starting a project with Angular or already using it, just stick to Angular, don't make things more complicated that needed.
If you are considering using React, then read about how to structure your application using Flux or a similar approach, because it's going to be completely different than Angular. Then choose the one you like the most (knowing that Angular2 is coming, with a design that is closer to that of Flux/React).