I'm not positively certain - but if I would have to guess, I'd say it's obviously to keep your mouth moist and to help swallow food better - because I think it would be hard to swallow any kind of food with a dry mouth.
i cant live without you
i cant live without u
you cant live without water or air
They can and did live quite happily without it.
They can.
insulin
Gravity
because
we cant
You Cant Live Without Your Liver!
All of them, really.
you cant