Yeast nutrient plays a very important role in helping promote proper yeast reproduction in your wine must. This ensures that your wines will ferment to completion with little chance of off-flavors occuring. Certain fermentable ingredients are very nutrient deficient including Honey (Mead),
and White Wines.
This means that when you pitch your yeast into your must, you need to use a good complete yeast nutrient to replace the nutrients that are lacking in the must. This is also true for wines made from fresh grapes. Wines made from fresh grapes which are naturally low in nitrogen will have a propensity to produce hydrogen sulfide (rotten egg) aromas that can be difficult to remove later.
In sum, if you are making fruit wines, white wines, wines from fresh grapes or mead, you should be adding a good yeast nutrient product at the beginning of the fermentation process. Some good examples are BSG Superfood and DAP (Diamonium Phosphate)