If you are trying to save money or the wine supply store is too far away, crushing up some Vitamin B1 helps give the yeasties some of what they need. When making wines with fruit other than grapes, some nutrients are lacking, depending on which fruits you choose for your primary..watermelon (since it's made up of mostly water) being the most dependent on needing supplements.
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.