Once at Sun I realized that it was more important for the company that the Unix world standardized on a single window system than that the standard be Sun’s NeWS system.

A user’s X environment consists of a set of client applications, an X server managing displays and input devices, and a window manager application implementing a user interface that allows the user to allocate the server’s resources such as windows to the clients.

One of the most amazing pieces of literature to come out of the X Consortium is the “Inter Client Communication Conventions Manual,” … It describes protocols that X clients must use to communicate with each other via the X server, including diverse topics like window management, selections, keyboard and colormap focus, and session management.

Source: DSHR’s Blog: X Window System At 40

X11 is important, even if it’s being displaced by Wayland