Why add sugar?

[splinterz169]

I see a lot of recipes for fruit wines that say you have the juice, and you add sugar before racking with yeast... I just wanna know why? From my understanding of the science behind this all, sugar will just turn into CO2 not alcohol, only the complex sugars turn into alcohol. Is it a preference? I like anything I make to be dryer anyways it's not like index the extra sugars and since it won't boost alcohol content... I dunno. Can someone explain this to me or is it really just preference?

 

[Yooper]

the gist of the science in fermentation is that the yeast "eats" sugar, and produces two by products- alcohol and c02.

The amount of sugar fermented directly translates into the amount of alcohol produced, and it's calculable. So if you want a 12.5% ABV wine, as an example, you can choose the amount of sugar to add to get to that ABV.

Simple sugars, like table sugar and corn sugar, are completely fermentable and will ferment 100%.