What is the use of Watchman while working on React?

Why do we use watchman when working on react js.

React Native uses watchman to detect changes made by you while writing react script and it automatically reflect on browser without refreshing it.

It is also helpful to remove error when you face while working on react like when you start react and it does not work on browser with the URL http://localhost:3000 and says site can't be reached then by installing Watchman you can resolve the issue and start working react js.