There are different yeast nutrients designed for application at different points in the fermentation process. Go-Ferm for example, is meant to be used during the yeast rehydration phase and consists of vitamins, minerals and amino acids (but with no DAP); Fermaid and Supervit, on the otherhand, are meant to be added prior to yeast inoculation and/or at 1/3 sugar depletion. They consist of DAP along with vitamins and minerals. It is a good idea to use yeast nutrient to ensure you have a healthy fermentation, but depending on your must, it may not be necessary.