Compared to ancient beer the yeast of modern beer is killed off before bottling, so you can't use it as a yeast replacement, but instead this allows the beer to be stored for a long time.
Some microbrews don't do this and you can recuperate live yeast from those beers for brewing or baking.
Some micro breweries use a finishing yeast that's different than the fermentation yeast to keep that proprietary. I think anchor brewing comes to mind for this.