what is X11 all about? what is the benefit of having X11?

It's for use of the old Darwin system. There is no real benefit unless you wish to run software developed only for use with Darwin.

Welcome to Apple Support Communities

If you ask this, you will not probably need it. X11 is a version of the popular X Window System, used by most UNIX operating systems (including OS X). Since OS X Lion (I think), it is no longer included, and you only need it to run some specific applications.

Read this:

https://support.apple.com/en-us/HT201341

Ciao.

http://en.wikipedia.org/wiki/X_Window_System