why does RC cache to the DB and local file?

In this code in func.inc, it looks like RC is caching a message both to the database and to the local temp directory. Why does it cache 2 times?

// let's cache the message body within the database if ($CONFIG['enable_caching'] && $cached && ($CONFIG['db_max_length'] -300) > $headers->size) { $DB->query("UPDATE ".get_table_name('messages')." SET body=? WHERE user_id=? AND cache_key=? AND uid=?", $msg_source, $_SESSION['user_id'], $cache_key, $uid);

return $msg_source;
}

// create dir for caching if (!is_dir($cache_dir)) $dir = mkdir($cache_dir); else $dir = true;

// attempt to write a file with the message body if ($dir && ($fp = fopen($cache_path, 'w'))) { fwrite($fp, $msg_source); fclose($fp); } else { raise_error(array('code' => 403, 'type' => 'php', 'line' => __LINE__, 'file' => __FILE__, 'message' => "Failed to write to temp dir"), TRUE, FALSE); }

return $msg_source;