Difference between addSubview and insertSubview in UIView class

The only difference is in where the view is added: whether it is the frontmost view (addSubview:), or it is before the 5th subview, (insertSubview:atIndex:) or if it is immediately behind another subview (insertSubview:aboveSubview:).

Using insertSubView: you can specify the index, which determines z-order of views. A view with a higher index lies above those with lower indices.