此代码和addChild（）有什么区别？

Question

这是我在一份Apple Swift教程中找到的代码：（在UIViewController类中）

private func attachChild(_ viewController: UIViewController) {
    addChild(viewController)

    if let subview = viewController.view {
        subview.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(subview)

        subview.widthAnchor.constraint(equalTo: view.widthAnchor).isActive = true
        subview.heightAnchor.constraint(equalTo: view.heightAnchor).isActive = true
        subview.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
        subview.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true
    }

    viewController.didMove(toParent: self)
}

我想知道调用attachChild和仅仅调用addChild之间有什么区别，或者换句话说，此函数的作用是什么？