Drupal: Are hook_ functions in *.api.php ever called?

In Drupal 7, every core module has a *.api.php file, where * is the name of the module. For example

modules/node/node.api.php
modules/path/path.api.php

What are these files for? They contain functions that start with hook_, and the name of a hook that (I think) the module invokes. For example

modules/system/system.api

has

function hook_entity_view($entity, $type, $view_mode, $langcode) {
  $entity->content['my_additional_field'] = array(
    '#markup' => $additional_field,
    '#weight' => 10,
    '#theme' => 'mymodule_my_additional_field',
  );
}

There's an entity_view hook that's invoked by the system which you may implement in your own modules, but (it doesn't appear) that ho开发者_如何学Pythonok_entity_view is ever called.

What are these function for. Are they ever called by the system? If so, when? If not, why are they there?

Nope, it's just documentation files that describe hooks by modules. About hook_entity_view: you can add it in custom module: YOURMODULENAME_entity_view(...).