It's useful when the client always has its own static IP that _doesn't change_ between sessions. In this case, where the public facing IP may be shared by thousands of users, it provides no real security. All you'd have to do to gain access would be getting the client IP and finding some way of getting on the same network. Which in many cases could be as easy as subscribing to the same cell network or other ISP, or connecting to the guest wifi network of an office building.