Our modern diet is so refined that we do not have to chew our foods very much to prepare it before swallowing. As a result, some foods are swallowed after less than ten chews. The act of chewing stimulates the muscles of our jaws, and stimulates our salivary glands to produce saliva. Saliva is very important in the prevention of tooth decay. Individuals lacking saliva (e.g. following X-ray irradiation of their tumour, affecting the salivary glands nearby) can get cavities in the most unusual places, for example on the tips of the cusps of their teeth. Saliva helps to neutralize acids produced by bacteria in dental plaque following ingestion of sugars, thereby preventing decay.
Chewing sugar-free gum after food can stimulate saliva flow; the gum also has a cleansing effect on the teeth. Chewing apples also has the effect of cleansing the teeth.
There are some individuals who chew so little that their opposing teeth from the top and bottom jaws do not fit properly together. The more you chew, the tighter your teeth will fit against each other. The tighter the fit, the more stable the teeth will be, due to the inter-digitation of the cusps of opposing teeth.
As we chew, adjacent teeth (in the same jaw) also move slightly and rub against each other, resulting in a very slow wearing down of the enamel. As a result of the wear, round convex surfaces of adjacent teeth touching each other become flatter. This slightly flattened surface presents a broad area of contact between adjacent teeth, so adjacent teeth do not slip past each other so easily, thus promoting stability.