how field attached to a node or entity ?

I watched this function many times, but still have some puzzles,

function field_attach_load($entity_type, $entities, $age = FIELD_LOAD_CURRENT, $options = array()) {
    //  
    //  
}

There is a snippet

  if ($cache_read) {
    // Build the list of cache entries to retrieve.
    $cids = array();
    foreach ($entities as $id => $entity) {
      $cids[] = "field:$entity_type:$id";
    }
    $cache = cache_get_multiple($cids, 'cache_field');
    // Put the cached field values back into the entities and remove them from
    // the list of entities to query.
    foreach ($entities as $id => $entity) {
      $cid = "field:$entity_type:$id";
      if (isset($cache[$cid])) {
        unset($queried_entities[$id]);
        foreach ($cache[$cid]->data as $field_name => $values) {
          $entity->$field_name = $values;
        }
      }
    }
  }

Why unset ($queried_entities[$id]);
and Why $entity->$field_name = $values; again ?

Can some one help us make it clear Thanks.
It s very clear that its a small part of this function. Then realized how difficult drupal code is.
God bless me.

Thanks and Regards.