I have been looking into UUCP (Unix-to-Unix Copy, a very old network protocol) as a way of communicating with some Linux boxes that don't always have an internet connection. While doing so on my Macbook (with El Capitan), I found entries in my /usr/share/uucp/sys file for uucpsys1.apple.com and uucpsys2.apple.com. (You can also see these by running uuchk | less.)
What is Apple doing with UUCP? I found this post on the Apple forums by someone who noticed a UUCP user, but no one came forward with an explanation, so I'm hoping someone here will know.
I'm especially curious because /usr/share/* falls under System Integrity Protection in El Capitan, which means I'll need to set up a separate UUCP implementation if I want to use it for my purposes — but it also suggests that, if Apple's shipping OS X with a UUCP that only they can use, they must be using it for something.
ETA: I've also found a launchd daemon at /System/Library/LaunchDaemons/com.apple.uucp.plist. It has <key>Disabled</key><true/> in the plist, so it won't be run by default, but other daemons could enable it programmatically without altering the plist. I'm also intrigued to see that it's in /System/Library/LaunchDaemons/ (and thus subject to SIP) rather than /Library/LaunchDaemons/ — it adds to my hunch that it's something Apple is using (or maybe was using, as patrix comments), rather than just a miscellaneous *nix tool they happened to include.