Wait, aren't the grapes (or, more specifically, the sugars in the grapes) supposed to be the yeast nutrient? Yes … but sometimes that's not enough. Yeast nutrients are often added to wines during fermentation to ensure that the desirable wine yeasts have enough of the right kind of food to promote a rapid start, steady reproduction and complete fermentation.
Used in conjunction with the right yeast strain, the right proportion of grapes to yeast and the proper fermentation temperature, yeast nutrients can help to ensure that the finished wine is not too sweet. Yeast nutrients are a must for non-grape wines (sometimes called "country wines"), such as peach or berry wines.