Some fruits are so high in tart acid that using 100% of the juice will make an undrinkable wine.
Wine grapes are just about the only fruit that provides enough sugar and low enough acid to make wine without adding sugar or diluting with water.
Some fruits, like raspberries, are wonderful to eat and make a good wine. However, if using 100% raspberries, the wine would be bitingly acidic and harsh, and there is not enough sugar to get above about 5-6% ABV in them. So sugar is added to boost the SG, and then water is added to help dilute the intense acids. Once the sugar in raspberries is fermented out, the results aren't satisfactory unless some of the acid is diluted. Then, it makes a lovely wine!