Why use Express with ReactJS

React configured as a Single Page App, renders the express routing all but unnecessary. The standard transition from a web server rendered app to a single page app is to change the server into a REST Web API and any server side data needed from the React App is obtained through AJAX calls.

So in most cases you don't need Express when you are using React to handle your routing except for in less common cases when you might want to use express as a reverse proxy or something.

If you want to do server-side rendering, you'll need a node server (typically but doesn't have to be express) to render your React components. This could not only help with SEO but also allow your page to appear to load faster (you wouldn't have to wait for the JS to download and run before the user sees the HTML).

If you don't care about the benefits, don't use SSR.

when deploying react application it is just usually an html and a JS file. So you need some kind of server to host it. Luckily there are services out there that offers that like S3, Github etc.You're using Express because it can host the html and js files. So technically you don't need specifically express but you need some server that will host your files.