wp_enqueue_style $dep argument does nothing?

The 3rd argument of wp_enqueue_style means when enqueuing this stylesheet make sure it is after the list of dependencies.

In your case, prior to enqueueing the child-style WordPress make sure that parent-style is enqueued first.

Even though the enqueued statements are in the correct order:

wp_enqueue_style('parent-style', get_template_directory_uri() . '/style.css');
wp_enqueue_style('child-style', get_stylesheet_directory_uri() . '/style.css');

There will be an instance where child-style is enqueued first before the parent-style. So, to make sure that the order of the enqueued stylesheets are correct you need to supply the third argument, which is a list of dependencies.