Gain graphical access
SSH allows you to connect to UNIX machines via a command line. If you are connecting using SSH from an OS that does not use an X11 window manager, you will need to install an X server to run graphical applications.
For security reasons XDMCP connections are blocked from the Internet. Many hosts have XDMCP disabled for the same security reasons. (XDMCP is the default way X11 communicates)