Why does the default Magento theme have /html subdirectory?

If you look at https://github.com/magento/magento2/tree/2.3-develop/app/code/Magento/Theme/view/frontend/templates , there’s phtml templates in this folder, but also in a subfolder called /html

Looking at the code style, there doesn’t appear to be any noticeable differences between the phtml files in templates and the ones in templates/html.

What’s the point of this folder? When developing a theme, should we be using our own html subfolder and if so, what is supposed to go in it (other than overriding / extending the default Magento 2 phtml files for replacement)?