Originally Posted by some spit authority

What is Saliva?
Saliva is a fluid of mostly water which is secreted out of three pairs (for a total of six) of salivary glands within the mouth.

These pairs are called the parotid, the submandibular and the sublingual glands. Indeed, these are the big three, responsible for every spit and every puddle of drool on one’s pillow in the morning.

Now, what is the chemical composition of saliva?

The stuff is mostly water (98%), and this portion of it is really somewhat inconsequential and no different from what comes out of the kitchen faucet.

Obviously, if saliva was only water, one might as well just have a bottle Disani laying around at all times for all the good it would do. Indeed, it's the remaining 2% that's the key to the importance of saliva.

Saliva Chemicals
This deceptively small portion contains several different chemicals – mucus, electrolytes, antibacterial compounds, and enzymes – all of which play their own particular role in one’s mouth.

The enzymes in particular are pretty crucial, and one of them specifically: Ptyalin (from the amylase family of enzymes) serves to break down the starches in the food we eat, beginning the crucial processes of digestion long before the food ever reaches the stomach. It would surely be missed if it was gone.

The small amount of mucus within saliva turns it into what’s called an alkaline chemical, meaning that it acts to neutralize acids in the mouth.

Why is this important? Well, when the urge is felt to vomit and the brain sends signals to the mouth to rapidly increase salivation, one should be grateful, because the acid from the stomach can be quite hazardous inside the mouth, throat and teeth. Saliva, in other words, protects the mouth from vomit.

Other Qualities
One could go on for some time – saliva acts to lubricate food in order to allow it to slide through the esophagus without any unnecessary discomfort, it solubilizes dry food which allows one to taste what is being placed in the mouth, and it aids in oral hygene by cleaning unwanted microbes and bacteria from the mouth (the fact that the salivary glands slow down production during the night is the reason for “morning breath”), and the list goes on.

As for licking wounds... saliva definitely has an antibacterial element to it (and even trace amounts of hydrogen peroxide), which does, in fact, make it a bit more effective at cleaning wounds than actual water.

In this light, perhaps this is one of its intended purposes that we simply don't use often enough.

Nerve Growth Factor
Additionally, it has been shown that in some animals (mice in particular), saliva contains a product known as NGF (nerve growth factor), which is actually a very capable healing agent for wounds.

It has been shown that a wound dosed in NGF heals up to twice as fast as a wound without it. Alas, such a substance has never been shown to exist in human saliva, but scientists haven't given up hope of finding it.

Saliva is not just a gift to animals, but it is an important one to humans as well, and one that is far too often taken for granted.