Sugars of various types are used for many reasons in brewing.
Fermentable sugars will increase alcohol, thin the body of a beer and held add some perceived dryness. These sugars are also typically used to bottle condition (carbonate) your beer.
Unfermentable sugars will not add any alcohol and are typically used to manipulate the body and dryness by increasing or thickening the beer somewhat. While these sugars can add some slight residual sweetness, it is not particularly noticeable.