For now, it allows the Twig extension to register the Routing Twig extension:

$app = new Application();
$app->register(new SymfonyBridgesExtension(), array(
    'symfony_bridges.class_path' => '...',
));
$app->register(new UrlGeneratorExtension());
$app->register(new SessionExtension());
$app->register(new TwigExtension(), array(...));

You can now use {{ path() }} in templates to generate URLs.

The order of registration of the extensions is significant.