The mucus in saliva is extremely effective in binding masticated food into a slippery bolus that (usually) slides easily through the esophagus without inflicting damage to the mucosa. Saliva also coats the oral cavity and esophagus, and food basically never directly touches the epithelial cells of those tissues.
Saliva coats the oral mucosa, mechanically protecting it from trauma during eating, swallowing and speaking. In people with little saliva, soreness of the mouth is very common, and the food (especially dry food) sticks to the inside of the mouth.