I believe it is to add more fermentable sugars to the wort. Thus, raising the alcohol content, without adding body. It could actually thin out the body of the brew, and that can be desirable in heavily malted beers like RIS, porters, etc.
p.s. some info on molasses
. I just started using muscovado sugar in my heavy ales, porters & stouts. It carries the molasses and brown sugar flavor as well as some extra minerals...