What is the purpose of nutritional yeast?

It's a flavour booster like Marmite (or Vegimite if you are an Ozzie)

Not to everyone likes, but it sure does has an interesting taste

Usually made from fermented barley

let's not forget the masses of B vitamins it contains too.

A paste applied to the cheeks will result in a flush due to the B rush. Not that I recommend trying it (old beauty tip from a health mag)

Salads use a sprinkle of flakes instead of nuts: lower in Cals

I use nutritional yeast for the taste. I think comparing it to a milder form of Marmite flavour, or even miso is apropos - it has a similar tanginess - in my opinion. When I choose something low caloric for a good flavour enhancer, this is a good thing to keep around.

A good introduction for you to try nutritional yeast might be to sprinkle it on popcorn, or add it to a tomato sauce, or even on buttery toast. Use it like a flaky spice.

Nutritional yeast is a valuable source of B vitamins, especially B12, for vegans. Recently it's had an image makeover, with cutesy names like 'nooch'

It's commonly described as having a "cheesey, nutty" flavor; more technically, it's a vegan source of umami, the glutamic acid flavor associated with rich protein sources. (Umami alone apparently generates a lot of discussion; see Delish Knowledge, LifeHacker, Swirled...)

Me, I'm not a vegan, or even a vegetarian, but I like to sprinkle it on baked potatoes and roasted onions, or add a spoonful into a hearty pot of soup.

Serious Eats talks about using nutritional yeast to make a vegan mac-n-cheese clone, sprinkling it on popcorn, and even as a dough conditioner for noodles.