The spigot makes life easier not needing to rack after fermentation.  Also, can serve as a bottling bucket and even a dispenser of noncarbonated beverages. 

Made of special PET that is BPA free, has a negligible oxygen permeability, and specifically designed for fermenting beer and wine. They are light weight, unbreakable, and will not stain or carry over flavors.

PET carboys use a #10 rubber stopper