Normal table sugar is chemically know as Sucrose. Sucrose is one part glucose and one part fructose stuck together, and cannot be used by yeast to make alcohol. They get around this problem by making an enzyme called invertase. This enzyme splits the sucrose into its constituent parts - glucose and fructose. Both of these can be converted into alcohol by our friends, yeast. Unfortunately, invertase tastes bitter. So, we get around this problem by using brewing sugar - also called Glucose (or for the technically minded Dextrose Monohydrate).

But is it really necessary? Well I suppose that depends on wether you are making a delicate floral wine, or an overpowering blackcurrant wine.

Finally, try using brewing sugar to sweeten your finished wine.  You will find that it has a very different taste profile to table sugar - it is much softer.