I think they just give your reins a little more weight and help keep your leather reins from getting wet if your horse drinks frequently out on the trail.
But most folks don't seem to use them anymore. I have never tried them myself, but I like how they look. I did have a friend who tried them, but she didn't like all the "sway" they created as the horse walked.
If they have any other purpose, I haven't heard about it. I think it basically keeps your leather out of the water.