The sugars in sucrose in its various forms (including candi sugar) are nearly 100% fermentable, whereas the more complex sugars from malted barley are not.
So the main thing sucros in any form does is create more food for the yeast but contributes to a drier finish than if you brewed the same OG but from all malt.
Clear candi sugar does nothing, IMHO, that plain table sugar wont do.
Dark candi sugar (or syrup) adds carmelly sweetness and complexity to dubbels, string dark ales, etc.
The relatively large amount of sugar in several styles of Belgian ales is what allows them to have high gravities and relatively low IBUs without being cloyingly sweet.
Bottle conditioning: Robust Porter
Drinking: Saison Dupont clone, tripel
Coming soon: Columbus APA, Rich Red ale