A key use-case of React would be a website with dynamic content. If your making a portfolio you could totally do without React. Facebook is built with React, and it is very dynamic, compared to your average website.
I would consider most if not all of React apps to be built in React, I see little to no reason to have only parts of the app in React. I can see React being integrated for specific parts of a large website, but I would see that more as a special case, rather than the norm.
A good way to look at React is as a way to build applications by using React to handle the UI in a clean straight forward manner. The key part is application, which usually requires dynamic data, UI, and content, all of which requires dynamic UI, which thus leads back to React.
PS. I am not a React guy I am an Angular guy, but they are both front-end frameworks (or React is a lib if you must call it that)