What is X11 used for ?

It is a windowing system (GUI) for apps that were written to work in X-Windows.

(Roughly) Just as DOS had Windows as its Graphical User Interface, so Unix has various GUIs, among them X-Windows, & OS X

more info on X11 (Now XQuartz) here:

https://xquartz.macosforge.org/landing/

An example of an app that only runs with X-Windows environments would be WireShark, a network analyser.

In addtion to what SilverSkyRat said, there are some users that need to interoperate with Unix/Linux systems, and having X11 available on the Mac has made our life so much easier.

However, for the normal Mac user, X11 is not very interesting.

Thank you. That clarified it nicely!