Heavy donut ring snaffle question....the purpose?

Nope! You are correct!

I personally have never used one to get a horse to drop its head or any other artificial aid so I can't give you my opinion!

http://www.equinecloset.com/dorisnbit.html

It places constant pressure on the mouth. That bit also looks to have a thin mouthpiece, and I would consider it quite harsh and would not ever consider putting that in a horse's mouth. I'd imagine they intend it to help keep the horse's head low, and discourage head tossing, since tossing the head with that sucker biting into their bars and tongue would hurt like heck. But any effects will disappear once the bit is changed, as, due to the weight, that bit will not provide for relief from the punishment when the head is in the proper position. Thus, it won't train the horse because you can't effectively tell the horse when they achieve the desired result.

Add: I have no problem with training a horse to have a low headset or with showing. I DO have a problem with so-called "aids" like this that accomplish those thinga by causing an animal PAIN. They're resorted to by lazy riders. You can train a horse without causing undue pain and discomfort. I've developed low western pleasure headsets in quite a lot of horses when called for over my life, with actual training and a plain snaffle bit.

You are correct, these are typically used to help teach a horse to drop it's head and give to bit pressure. Personally, I don't agree with some of the comments "downing" this type of bit. I think that there is a place for a lot of different training aids like this, if used correctly and by quiet, calm hands. Obviously, the mouthpiece is on the thinner side, so it would be more severe than a regular snaffle bit, and with the extra weight of the rings, you have to be careful not to "jar" the horses mouth with a lot of heavy handed see-sawing, etc.

Just like a martingale, german martingale, correction bits, draw reins, etc., the heavy donut snaffle is used to achieve a certain goal, and is not meant to be used on a constant basis. I mean, western pleasure riders use "weighted reins", reiners (which is what I do) put slide plates on their horses for longer stops...all of these things have their place if they are used properly and for a purpose.

There are a lot of people who are not into "competition" who believe that horses should be allowed to be as "natural" as possible, but the simple act of riding them is unnatural. Putting a halter on their heads and tieing them to a rail to brush them is unnatural. Keeping them in a corral and not turning them out on 5000 acres to be free and fend for themselves is unnatural. My opinion is that as long as you are not being "cruel" to the animal, horses are meant to do a job. Sure, we bond with our horses, and I love mine to death, but my reining mare has a job and she does it well. I care for her better than most people I know care for any of their pets, and I am very particular, but I realize that she is meant for a job and she knows that as well. She completely blooms when she is out in the arena running a pattern, and it's obvious that in all of her training (she was a futurity horse as a 4 year old) she has learned to accept and love her job.

Don't let people get down on you for PROPERLY using training aids. Just be kind, careful and competent in whatever you do with your horse. :)

You can find you answer in the description in the link.