What's the point of using xauth when using X11 forwarding?

[Xu Pu Ti]

I still don't understand the point of authenticating myself to my own local
X server when using X11 forwarding, I tried:

  ssh -R /tmp/.X11-unix/X0:/tmp/.X11-unix/X0 user at server
  # and then
  DISPLAY=:0 xterm

and everything is working fine without the mess with xauth, so why it is
required to use use xauth when doing X11 forwarding with ssh?