A child theme is ideal when you add custom CSS to your website. Most of the time when a developer updates the templates your css will be gone. To solve this issu it is advised to build a child theme. A child theme is a coppy of the main wordpress theme where you can make changes to the css without it getting lost in an update.
There are more ways to make add a child theme to your website. The author from the video above has just shown the easiest way to add a child theme. You can also chose to add a child theme through FTP. This is a more complex way to add a child theme to your website.