Why `method_exists($scope, 'extend')` exists in method `withGlobalScope` of Laravel's Buidler

When I read the Laravel 5.3's source code, I find the following codes

public function withGlobalScope($identifier, $scope)
{
    $this->scopes[$identifier] = $scope;

    if (method_exists($scope, 'extend')) {
        $scope->extend($this);
    }

    return $this;
}

But the creation of GlobalScope is as followed

public static function addGlobalScope($scope, Closure $implementation = null)
{
    if (is_string($scope) && ! is_null($implementation)) {
    return static::$globalScopes[static::class][$scope] = $implementation;
    }

    if ($scope instanceof Closure) {
    return static::$globalScopes[static::class][spl_object_hash($scope)] = $scope;
    }

    if ($scope instanceof Scope) {
        return static::$globalScopes[static::class][get_class($scope)] = $scope;
    }

    throw new InvalidArgumentException('Global scope must be an instance of Closure or Scope.');
}

I am confused about why it is the following codes in the method withGlobalScope

if (method_exists($scope, 'extend')) {
    $scope->extend($this);
}

Thanks!

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