Thanks to Signal Pro Ashley for this question!
It's amazing how there's always something more to learn.
Just when you think that you've amassed a truly encyclopedic knowledge of cables, home theater, and the whole geeky lot, there's something else that creeps up that you didn't know. Perhaps today you woke up not knowing what "messenger" wire was, or perhaps you thought it maybe had something to do with a telegram. (Pause while younger readers google "telegram.")
is a second cable that's bonded to regular cable like the image above. It doesn't carry any signal and is just there to add strength and rigidity to a cable. That extra strength could be needed to keep a cable from sagging when run through the air or make it easier to fish over long distances in commercial installations. Essentially messenger wire provides the backbone so that the cable carrying signal isn't strained.
You'll actually see messenger wire used on telephone poles a lot, but you are a DIY'er would probably use it to avoid burying the cable because of local ordinances or because let's be honest, you just don't want to bury it. If you are going to string wires up in the air, using messenger wire is a pretty good idea.
By the way, you should never use messenger wire for grounding. If you do want to have a ground wire bonded to the cable, Solid Signal sells that as well
, but the messenger wire itself isn't designed to carry current. It's better to use ground wire for grounding and messenger wire for support.