Lots of different sugars can be used in brewing — why do you ask would we want to use anything other than the sugars that come naturally form the barley?
Well there are a few reasons that may apply in some circumstances:
a.) To raise the alcohol level without increasing the body of the beer.
b.) To lighten the body of the beer while maintaining the alcohol level.
c.) To add some interesting flavors.
d.) To prime the beer for carbonation.
Source: John Palmer - How to Brew
