Sugar in Belgian Beer rises the abv and dries out the beer, it helps contribute to that "crispness" belgians have. Strong alcohol without the heavy body of beers of similar gravities. And the different grades of candi sugar Light to dark contribute a bit of flavor. But really not much, since most of the sugar ferments away leaving the caramel notes behind (like using honey or brown sugar).
But candi sugar is nothing scary or really even something you need to buy. Many of us make our own quite easily.
All Belgian candi sugar really was/is is the cheapest sugar they could buy in bulk (usually beet as opposed to cane, because sugar beets are local, but any sugar will do) boiled to the right color and consistency.
And in the old days it would vary from order to order, it was just what they could find in bulk cheaply, and used as a cost cutting adjunct more than anything. But it just gives Belgian beers their character.
It's the same thing as making CANDY....everything from soft caramels to hard candy, depending on how much you use and how long you boil it.
Here's several recipes;
http://www.franklinbrew.org/brewinfo/candi_sugar.html
You don't really need to order and over priced "special" Belgian sugar, you just need a 5 pound bag of whatever is cheap at the grocery store, a pan, whatever extra ingredients the recipe might call for (usually an acid or cream of tartar) and a candy thermometer, usually in the same aisle as the bag of sugar in the grocery store.
If you can boil beer, you can make your own.