(From the packaging)
"Brewers' Yeast Powder contains Vitamins B1, B2, and Niacin, together with other traditional components and B-Complex factors found in yeast."
If you're at all deficient in B vitamins, supplementing diet with brewers' yeast will have a marked effect (hence the high ratings). The taste of this product is not the best, but it's also definitely not the worst. (The 'debittered' notice on the packet is accurate!) The flavor is strongly yeasty, slightly nutty.