I have several recipes for seitan which call for nutritional yeast. I've never used it before and am curious about its purpose.
Is it for flavor, texture, nutritional value?
This product is new to me, but it looks interesting.
The flavour of nutritional yeast is described in Wikipedia:
Nutritional yeast has a strong flavor that is described as nutty, cheesy, or creamy, which makes it popular as an ingredient in cheese substitutes. It is often used by vegans in place of parmesan cheese.
Nutritional yeast is a so called complete protein.
A complete protein (or whole protein) is a source of protein that contains an adequate proportion of all nine of the essential amino acids necessary for the dietary needs of humans or other animals.
This also explains why it is popular among vegans, that needs to replace the amino acids in meat with alternative food sources.
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.
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.