Isn't it cool when someone puts their dog through a string of behaviors without uttering a word? It is a goal of many to do just this, work silently. The reason that it is impressive is that your dog must be paying close attention to be able to follow hand signals. If they aren't watching then they aren't going to see the signals right? Attention can be the most difficult thing to teach when trying to use strictly hand signals; especially when you add distractions to your training.
In fact most dogs learn hand signals before they ever learn a verbal cue; most but not all. Elsa learns words just as fast as she learns hand signals. Because of this I have used the words on their own and am now adding hand signals. Yesterday I taught her the swing; the action of going to heel position from sitting in front of me, going all the way around to the right of me and ending up on the left. It took her all of 5 min. from start to finish. I never used a verbal cue for this because it is a lured behavior. Now that she is reliably performing it I will add a verbal cue and slowly remove the hand signal. This allows you to use the verbal, visual or both.
Hand signals are extremely useful for keeping a dog's attention on you, they learn to go about their business with one eye on you just in case. It is also extremely helpful as our dogs age and lose their hearing. I have always used hand signals with my old guys once their hearing starts to fail. It is much easier on the nerves than yelling in hopes of some sort of acknowledgement. It is sad when an old dogs vision goes as well and you are left with only touch to communicate.
Many hand signals are in fact shortened versions of how we originally teach our dogs a behavior. Over exaggerated signals can be shortened up until they are barely visible. Dogs are masters at visual cues and the slightest movement from us can hold a great deal of information for a dog. When I watch and guide clients through the teaching of their dog; I often see visual cues that their dog is picking up on that they don't even realize. I will ask the owner to stand perfectly still, with their hands by their side and then use a verbal cue. Typically their dog will stand in front of them stupefied, which then leaves the owner wondering what's going on.
A turn of the head, a slight bend, motioning with the body or facial expression can be all that is needed for a visual cue. If you really want to know if your dog "gets" a verbal cue all on its own then you need to take away all visual cues completely. Standing like a statue you can then offer your verbal cues to see if your dog is listening. Dogs do a great deal more watching than they do listening so having a dog that will go through their paces strictly on verbal cues is just as impressive if not more so than a dog who will act on visual cues.
I believe all dogs deserve to learn; learning opens up doors to more learning once those doors have been opened. If you've got a real smarty then it can be difficult to think up stuff to teach them; but the more you teach the more means you are developing in communicating with your dog.