Brown sugar and Russian Imperial Stout

I am considering brewing a Russian Imperial Stout. The recipe calls for brown sugar and molassis.
AFAIK brown sugar is essentially cane sugar without the molassis removed.

What difference will it make if I add cane sugar (and molassis) instead of the brown sugar and molassis ?
What effect does the added sugar have besides a higher abv ?

sometimes adding too much cane / beet sugar can give you a cidery taste in addition to higher abv.

Only if added in quantities above about 25% of fermentables, and only when not inverted by acids. If sugars are boiled in the wort as they (usually) should be, they'll invert and the cidery taste won't be present. I've made strongs with 20% white sugar and with no problems at all.

As for molasses, I love it in stouts and porters. Be advised, though, that I've found it necessary to age them longer, as a youngish beer with any significant amount of molasses has a fairly assertive flavor, in my opinion. (I used dark, but not blackstrap, molasses).

Whats the reason for the added sugar ?

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.
--LexusChris

p.s. some info on molasses and muscovado. 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...

Muscovado is a good idea. The only problem with keeping it in the house is I end up eating it out the jar - that stuffs delicious.