When should I use 'open' event in Node.js?

Question

I'm learning Node.js and while touching events and streams topic I encountered this example:

const http = require('http');
const fs = require('fs');

http
  .createServer((req, res) => {
    const fileStream = fs.createReadStream('./some-data.txt', 'utf8');
    
    fileStream.on('open', () => {
      fileStream.pipe(res);
    });

    fileStream.on('error', (err) => {
      res.end(err);
    });
  })
  .listen(5000);

While I understand the logic behind this code (server transfers some big data in chunks with the help of streams), I don't understand the benefit of using 'open' event. Same code, where I just pipe data from read stream into write one (res object) right after creating fileStream without listening to 'open', produces exactly same result. What is the difference and in which cases should I listen to 'open'?

const http = require('http');
const fs = require('fs');

http
  .createServer((req, res) => {
    const fileStream = fs.createReadStream('./some-data.txt', 'utf8');

    fileStream.pipe(res);

    fileStream.on('error', (err) => {
      res.end(err);
    });
  })
  .listen(5000);