I'm assuming that the granulated sugar in the bag is easier to work with and won't turn into a hard block. It's also more expensive than the boxed kind. Are there any other differences, e.g. in the taste or texture after baking?
Your question seems to be conflating the question of bag vs boxed brown sugar with the question of "Brownulated" light brown sugar vs. regular light brown sugar.
Brownulated pours like white sugar, but turns into a paste when it gets damp or wet. I think it tastes funny.
Boxed vs Bag sugar is just a matter of storage and convenience. It's the same sugar.
I have a very sensitive sense of taste and smell. I think the Brownulated sugar has a slight chemical smell and taste to it but no one else in my family can smell or taste the difference. I suspect what I am picking up is some sort of chemical change that occurs during the processing. I won't be buying it again.
Brownulated sugar? Ah! "Brownulated sugar" is a registered trademark and sold in the US only.
From their (Dominos) web page:
This easy-to-measure brown sugar is a cup for cup replacement for regular light brown sugar. Convenient, great tasting, and easy to use.
So it must taste like ordinary light brown sugar after baking.
There is a very small amount of moisture in brown sugar, and a very small amount of acid in the molasses. I don't know what the chemistry of "brownulated" is exactly.
However, in practical recipes it makes very little difference.
If you measure by weight and not volume, it all becomes nearly moot.
Personally, I weigh all my sugar, using 7-8 oz per cup for volume based recipes. I just poke the brown sugar from its bag into the weighing bowl without trying to well pack it into a cup or anything, until the weight is approximately close to right, then bring the total sugar level up to the recipe level with white sugar.
Most home use recipes are incredibly tolerant of the tiny variation in the amount of moisture. Your personal preference can then dictate light versus dark versus white sugar, or the ratios among them. In most recipes (where the minor acid factor from the molasses on brown sugar isn't playing a factor), the only real issue is the total amount of sugar.
(For purists, yes I know 7-8 oz is a 15% variation--this is well within the tolerance for most recipes. If I think the recipe is written quite carefully, I use 7 oz which is a a better conversion; if I think the recipe is tolerant, or just for the brown sugar part, I might use 8 oz because the bags are usually a multiple of 8 oz.)
When measured in this way, regular brown sugar is just as convenient as "brownulated" would be, and I don't see any point in paying extra.