This article explains the role of the X Window System when it was first developed in the 1980s, and today. I highlight three advanced traits:

X was highly portable, so that applications written for X could run on virtually any Unix system, on BSD, on GNU/Linux, and on the Mac.
X allowed distributed computing. You could run graphical applications hosted on another computer, displaying them on your local desktop.
X was customizable to an almost limitless extent. This made X a platform for sophisticated interfaces such as KDE and GNOME.