Template hierarchy override

To understand how this works, take a look at the source file template-loader.php. Here you see several lines of statements which determine which template WordPress will load. Near the end is the filter, which allows you to undo all the previous lines.

For instance, one of the lines determines which template to load if is_single is true. You could use the filter to add extra conditions, for instance to load a different template if is_single is true and the post is in a certain category. Beware, this still means you would need to have a template file in your child theme (where you also have the filter in fucntions.php).

Basically, for all templates that WordPress can find through the conventional naming of the template hierarchy, it doesn’t make sense to use the filter. You would only use it if very specific conditions apply under which the template must be called.