I have not seen anyone claim that you will ferment those leftover sugars by adding simple sugar. Merely that apparent attenuation will increase, which will increase the alcohol, which will increase the perception of dryness. FG without knowing OG is a very difficult means by which to assess how dry a beer might be. Apparent attenuation can give you a really good idea.
A pound of sugar in my example would/could increase the abv from say 6% to 7%, which I would say is significant(I'm a lightweight), and would create a different beer- drier for instance. Hence the use of simple sugar to help "dry out a beer".
Am I missing something here?
i don't perceive a 7% beer with a 1.015 FG as any drier than a 6% beer with an FG of 1.015. in fact, I find higher alcohol content to taste sweeter rather than drier. I have at home a wheat wine that started at 1.104 and finished around 1.007 because of 3 lb of simple sugar. it tastes quite dry indeed but the alcohol still provides a very nice sweetness even as it evaporates off the tongue.
what we are trying to say is that a 6% beer made with all malt and a 6% beer made with malt and table sugar will have significantly different perceived dryness/sweetness. While a 6% beer and a 7% beer can taste quite difference because of the alcohol, if the extra alcohol comes from table sugar alone the perception of sweetness will not change significantly.
Adding simple sugar to a recipe is good for
a) bumping gravity and therefore ABV with minimal flavor/body change. this is the case when you simply ADD sugar to your recipe.
b) lightening body without change in ABV. this is the case when you SUBSTITUTE sugar for some of the malt as in a belgian beer.
take a belgian blonde around 6% and compare it to an american amber at about 6%. It will seem as if the belgian blonde is much lighter and dryer. More digestible as the belgians put it. while the Amber will be quite sweet and malty in comparison.