The frequent use of fake tails in the hunter ring is sometimes used as an example of hunters being about prettiness instead of about the performances of the horses. I will explain how fake tails can actually be used to balance out some horses in order to give a better performance.
Horses, like people, are born with different qualities of hair. Certain horses have a huge tail, no matter how you treat it, while others will always have a thin, airy tail. We're used to looking at the horse as a whole, tail included. This means that the volume of the tail can affect how we see the horse's balance.
A horse with a long or thick neck can look very front-heavy if there isn't enough tail to balance the picture. Similarly, a very short neck can look even shorter if there's a huge tail making the horse's body look even longer in comparison. Part of good turn-out for a horse show is learning what to do to give each individual horse the best overall picture. That is not to say that a judge cannot detect each horse's conformation anyway, but first impressions are everything. If you enter the ring looking perfectly balanced, you're already ahead and your terrific round will raise your standing even further. If you enter the ring looking unbalanced, your great round will have to first make up for that impression rather than add to it. That initial doubt in the judge's mind isn't likely to disappear completely.
This is especially important in an under saddle class when the judge has only seconds to look at each horse before narrowing the field down to the real contenders. If that first impression isn't good, you aren't likely to have a shot at a ribbon.
Most people aren't too worried about a horse's tail looking too thick. If your horse has a shorter neck and a thick tail, braiding the tail usually works very well to lighten the back end. The same amount of hair can look very different when the braided section breaks up the mass of body and hair.
I've modified a photo that I took at a horse show to illustrate why you might want to use a fake tail. Earlier I mentioned the usefulness of a fake tail on a horse with a long or very thick neck. These photos show another scenario in which the horse's head looks very large compared to the body.
The first photo is the original. This horse does not have a very thick tail, but it is also not abnormally thin. Your first impression might be that the horse is dragging himself a bit on the forehand. Now look at the photo for a bit longer. He's actually tracking up and using himself quite well. His head is quite large compared to the rest of his body (especially the neck, which it is most likely to be compared to due to its proximity), and this makes him look unbalanced at first, even if he actually is working from behind. The point of the working hunters is not to penalize a horse for having a large head, and by focusing on turn-out we can prevent that from happening.
This next photo shows what would happen if the horse's tail was very thin. The front-heavy effect is multiplied, even though the rest of the horse is identical. When you look at this photo, chances are that you will look first at that head.
The following shows the exact same photo, but with a thicker tail. I wouldn't consider this tail to be overly thick; in my opinion, it gives the horse just the right balance. Compare this photo to the one above and ask yourself which horse you would prefer if you only had a few seconds to compare them.
The last photo I will show you isn't all that useful for this particular horse, since he benefits from a thicker tail anyway, but it makes for an interesting comparison. This is a rough drawing of what this horse might look like with a very thick, loose tail. The look is not as refined as it is with the braid, but it really changes the first impression of the horse's balance. Take notice of where your eye is drawn to in this photo. Odds are, your eye will be drawn further back, taking in more of the horse.
When you're preparing for the show season, try to look at your horse's conformation objectively. Seeing your horse ridden in a video or by another rider can sometimes allow you to notice things about the picture he presents that you might not otherwise see. Choosing whether or not to braid the tail, and whether or not to use a fake tail, is more about your horse than about being pretty. Part of succeeding in the show ring is learning how to give your horse the best shot at winning before you even step into the ring.