There are a few theories on the licking and chewing performed by horses in exercise. The action is linked to relaxation, in simple terms and an emphatic trainer will understand the signs of relaxation in training, including the lick and chew. The French mention that it is not merely a lick and chew, but a swallowing action on the bit, which is a sign of acceptance of the bit. Interestingly, a horse does not produce saliva if they are not chewing. Humans produce saliva constantly. Saliva helps to neutralise the build up of stomach acid in a horse, so going long period of time without salivating, swallowing or eating, we are putting them at risk of stomach ulcers. A stressful tacking up period, followed by a stressful 15min warmup on the lunge with mouth tightly shut, followed by a tense riding session, followed by a lengthy tacking off period and perhaps a hose down without access to grazing, for example, could easily add up to 2 hours without any food and minimal licking, chewing or swallowing. This in turn causes anxiety and a cycle starts.
In many natural horsemanship programs, the trainer will watch for a lick and chew to ensure that their horse is processing information and is mentally and physically okay to continue with the session. Classical French Equitation insist on the relaxation of the horse as a foundation on which to build all training. It is mentioned in traditional dressage tests, but has been removed from the German Training Scale. With this loss of the absolute foundation of training any horse in any discipline, it may become easy for riders who are new to the art of equestrianism to overlook relaxation completely. For the good of the horse, we cannot allow this to happen. The only way to prevent this, is to educate.
The below link describes the physiological aspects of the lick and chew quite clearly: