I've just installed XQuartz on my Macbook Pro Retina 13" which runs El Capitan, because a development tool I'm using required me to do so. I know that XQuartz is (kind of) like the new X11, supported by Apple through an open source community, but I'm confused as to what the relationship is between XQuartz and El Capitan.
Here is what I'm thikning. If I had to install XQuartz by hand, then clearly El Capitan was using some other windowing system. But what? Having now installed XQuartz, what's changed? Have I downgraded my El Capitan in any way? I haven't noticed anything different yet. Is it reversible?
By the way, I haven't yet set up time machine, and I now regret it.