This has been done since god was a cowboy...the roller gives the horse something to roll in its mouth with its tongue, keeping it occupied instead of "fighting the bit". It works very well on a horse that constantly throws it's head around while being used.
Kinda off-topic but 20 years ago I took a welding course with a guy who had one of those remaining acreages down by COP. He made authentic western tack for Hollywood movies. His whole business was forging and blacksmithing on his property and FedEx'ing stuff down to California. Pretty cool way to make money from a hobby. He was a big inspiration to me. He fixed a old horse brand that had been in our family for years. Taught me some cool things about horses and blacksmithing too but I promptly forgot it.