setRedirectUri有什么影响？

I try to use google API php library PHP for oauth, and the official code as the follows:

$client = new Google_Client();
$client->setAuthConfig('client_secrets.json');
$client->setAccessType("offline");        // offline access
$client->setIncludeGrantedScopes(true);   // incremental auth
$client->addScope(Google_Service_Drive::DRIVE_METADATA_READONLY);
$client->setRedirectUri('http://' . $_SERVER['HTTP_HOST'] .'/oauth2callback.php');

But I found the downloaded 'client_secrets.json' already contains redirectURI, and I found the oauth in the app can work without calling $client->setRedirectUri :

$client = new Google_Client();
$client->setAuthConfig('client_secrets.json');
$client->setAccessType("offline");        // offline access
$client->setIncludeGrantedScopes(true);   // incremental auth
$client->addScope(Google_Service_Drive::DRIVE_METADATA_READONLY);

so my question is, what is the usage to call

$client->setRedirectUri

?

Google's OAuth 2.0 server authenticates the user and obtains consent from the user for your application to access the requested scopes. The response is sent back to your application using the redirect URL you specified.

$client->setRedirectUri('http://' . $_SERVER['HTTP_HOST'] . '/oauth2callback.php');

The page on your site oauth2callback.php must be configured to handle the response from the oauth server.