To me, it seems to give it a subtle, smokey flavor and enhances the affect of the herbs and spices.
I've heard of using cocoa, but I've not ever tried it.
It just sounds....ICK!
LOL!
Try just a little tiny bit of coca and see what you think. I would not make chili without cocoa. But I'd sure eat someone else's chili any way they cooked it! I am getting so hungry thinking about chili! I just love it after it has been in the refrigerator for a couple days. I always go back to my childhood and put saltine crackers in it.