TL;DR

Headless Chrome is a way to run the Chrome browser in a headless environment. Essentially, running Chrome without chrome! It brings all modern web platform features provided by Chromium and the Blink rendering engine to the command line.

Why is that useful?

A headless browser is a great tool for automated testing and server environments where you don’t need a visible UI shell. For example, you may want to run some tests against a real web page, create a PDF of it, or just inspect how the browser renders an URL.

https://developers.google.com/web/updates/2017/04/headless-chrome