Just the UI: Lots of people use React as the V in MVC. Since React makes no assumptions about the rest of your technology stack, it's easy to try it out on a small feature in an existing project.
Virtual DOM: React abstracts away the DOM from you, giving a simpler programming model and better performance. React can also render on the server using Node, and it can power native apps using React Native.
Data flow: React implements one-way reactive data flow which reduces boilerplate and is easier to reason about than traditional data binding.