Why does create-react-app creates both App.js and index.js?

Great question. The answer is that App.js is not necessary and I personally delete it and just use Index.js as the root.

People "say" it makes it look nicer / easier but it is just adding an extra step that is not necessary. I hope that they will eventually delete App.js out of npx create-react-app and just use index.js.

Edit: I'm not going to alter my original comment, however, I gave up deleting App.js. I now just funnel everything through App.js and use Index.js. The nice thing about index.js is you can do all your imports there and funnel them through App.js

Yes, App.js is not necessary. You can have just the index.js as follows.

// Import React and ReactDOM Libraries.
import React from 'react';
import ReactDOM from 'react-dom';
import CommmentDetail from './CommentDetail';

function getLabelText() {
  return 'Enter Name: ';
}

// Create React Components 
const App = () => {
  const buttonText = {text: 'Submit'};
  const buttonStyle = {backgroundColor: 'blue', color: 'white'}; 
  return (
    <div>
      <label className="label" htmlFor="name">{getLabelText()}</label>  
      <input id="name" type="text" />
      <button style={buttonStyle} >{buttonText.text}</button>

      // You can have other components here as follows.
      // CommmentDetail is someOther component defined in another file.
      // See the import statement for the same, 4th line from top

      <CommmentDetail author='Nivesh' timeAgo='3 months ago at 4.45 PM' commentText='Good Point' } />
    </div>
  )
}

// Take the react component and show it on the screen
// ReactDOM.render(<App />, document.getElementById('root'));
// You can use the following as well.
ReactDOM.render(<App />, document.querySelector('#root'));

import React from "react";
import ReactDOM from "react-dom";

function App() {
  return (
    <div className="App">
      <h1>Hello</h1>
      <h2>How are you!</h2>
    </div>
  );
}

ReactDOM.render(
  <React.StrictMode>
    <App />
  </React.StrictMode>,
  document.getElementById("root")
);

This is a sample where we can directly implement without using App.js.

App.js is the "top component" that contains the logic of your Application whereas index.js is the "entry point" for your server and contains the logic of the server.

This can also help to have multiple Applications if needed to switch from one to the other without changing anything in the server.

index.js in not necessary at all. It is just a convention to have both app.js and index.js for maintaining cleaner code. You can use any react componenct inside index.js file. The code snippet I have attached is an example of it, where I am using DateCounter component directly inside index.js file.

import React from 'react';
import ReactDOM from 'react-dom/client';
import './index.css';
import DateCounter from './DateCounter';


const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(
  <React.StrictMode>
    <DateCounter/>
  </React.StrictMode>
);

create-react-app use a plugin for webpack that name is html-webpack-plugin and this plugin use index.js for entrypoint like below :

const HtmlWebpackPlugin = require('html-webpack-plugin')

module.exports = {
 entry: 'index.js',
 output: {
path: __dirname + '/dist',
filename: 'index_bundle.js'
},
 plugins: [
new HtmlWebpackPlugin()
 ]
}

this plugin use for generating html file.