As oppose to plain Javascript? Or why is it in the title?
The former can be asked of any library / framework. Visit React's website to learn about why it's useful. Others have covered some reasons: it makes reasoning about applications easier and makes falling into composition of (mostly) stateless components the default
The latter is because people might be interested in seeing what's possible in React. Even viewing the page in React's dev tools is interesting and shows the author is also using Redux.
The former can be asked of any library / framework. Visit React's website to learn about why it's useful. Others have covered some reasons: it makes reasoning about applications easier and makes falling into composition of (mostly) stateless components the default
The latter is because people might be interested in seeing what's possible in React. Even viewing the page in React's dev tools is interesting and shows the author is also using Redux.