What is the purpose of nutritional yeast?
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.