Sanctum issued token

Hello there! I don't understand how sanctum uses the token expiration parameter. It seems to never consider it. I mean, if I change the expiration value In the config/sanctum.php file, Sanctum doesn't set automatically the expires_at date when it saves token in database:

   /*
    |--------------------------------------------------------------------------
    | Expiration Minutes
    |--------------------------------------------------------------------------
    |
    | This value controls the number of minutes until an issued token will be
    | considered expired. If this value is null, personal access tokens do
    | not expire. This won't tweak the lifetime of first-party sessions.
    |
    */
    'expiration' => 120,

The only way I found to set an expiration is to force it by myself when I issue tokens like that:

$token = $user->createToken('my-token', '*', now()->addMinutes(config('sanctum.expiration')));

Can you guys tell me what's the purpose of that expiration setting? many thanks.