I do quite a bit of advanced customization on my WordPress websites. This often includes creating plugins or editing the site template files in order to take the site to the next level. I recently did work on a website for one of the public relations and communications agencies in Chicago. The owner wanted a lot of custom things done on the website. It was necessary to edit some of the theme files such as the page.php, header.php and a lot of the css styles. Of course the website had no child theme.
If you are familiar with WordPress then you know that if you ever edit the theme files like this then they will be overwritten if the theme is ever updated. If you have done a lot of customization on the website then a theme update can destroy your site. However if you use a child theme instead then this will save your customized changes. I use a child theme for all of my projects and if I take over the management of an existing site then setting up a child theme is one of the first things I do. It only take a couple of minutes and it can prevent hours of lost work.