Yeasting DetailsWhy add yeast?
First, you don't need to add yeast. Most people add yeast because they believe their favorite strain will give them better flavors than otherwise, or that their preferred strain won't be as likely to stick.
If you believe your yeast strain is important to your wine's character, then add that yeast. If you want to save a little money and want to tell customers and wine writers that you practice native fermentations (you modern-day cowboy, you)... then you might let nature take its course and not add yeast.
But why is yeast important? Because it converts sugar to alcohol, and that is a good thing. And grapes are covered with yeast (good and bad strains), and your equipment is probably covered in yeast (good and bad strains, even though you cleaned it.)
Even if you're a native yeast fermentor, you should be familiar with yeast rehydration enough to fix any stuck lots you might have (and have the name of a strong bayanus strain to call in case of emergency.)