For humans the primary purpose of saliva is to pre-digest food and help make it easier to swallow before handing it over to the stomach.
For the cat, it does a great deal more. We know that a cat grooms herself with her tongue, saliva, teeth and legs. But the grooming process is more than just cleaning away particles from the surface of the fur.
Licking the fur helps to keep it smooth. Smooth fur acts as a more efficient insulation for the cat than ruffled fur as it is better at trapping the warm air around the cat's body. This keeps her warmer in winter.
Saliva also keeps her cooler in summer. Cats don't sweat. When she licks her fur the saliva gradually evaporates thereby acting in the same way as sweat, cooling the body through the scientific principle of the latent heat of evaporation.
Saliva also allows her to get her daily dose of essential Vitamin D. Vitamin D is important as it promotes bone growth, regulates Calcium and Phosphorus and promotes immunosuppression.
When she licks her fur she ingests Vitamin D which has been produced by the action of sunlight on her fur. Cats often groom more in the sun.
I talked about the cat's version of "scratching our heads" in another article. Another form of displacement therapy (an action which diverts attention from something that makes the cat feel uncomfortable) is licking fur.
A cat will also lick fur to taste her human companion who has just touched or stroked her. This is comforting for the cat; a bonding and communication process.
When grooming a cat will nibble at her fur when licking is insufficient. This process of tugging and disturbing the hair stimulates the secretion of oils from the hair follicles which oils the fur and makes it more waterproof.
When we stroke our pets they particularly like it when we touch those areas that cannot be reached by them (e.g. under the skin) as the act of stroking,for the cat, as if another cat is grooming her.
Yu see Cats are better in so many ways than humans :)
Picture Reproduced under creative commons (I'm sorry I've lost the Flickr name for the credit)