Is X11 still used today?

I have had X11 installed on my Mac for quite some time (as long as I have owned it I think) and have never used it. Do applications still use X11 or is out outdated at this point? Should I "learn" how to use it because I may use it in the future? The reason I ask is that I read that you can run X over SSH and I thought it might be useful to run a window manager to get some graphics from a remote machine. Then I realized I have never used X11 and launched it only to become confused. So should I use X11 and why?