title_body: What are the drawback of not using "brewer's yeast" when brewing beer?
upvoted_answer: Short answer; you get a lot of the flavour from the yeast, you can get a fairly big difference by varying the yeast in your recipe.
  
  For example, a Saison and a Pale ale have similar ingredients, but taste quite different, a Saison yeast imparts a considerable amount of flavour... likewise if you use a lager yeast, and properly lager your beer, you can get a different flavour again, without modifying any other part of your recipe.
  
  Lots of discussion on HomeBrewTalk forums, and other places. For a fun experiment, brew split batches and try two different yeasts to compare!