What happens to dough when sugar is added? I've been experimenting with a brioche recipe. After mixing the first few ingredients (flour, liquids, etc.) and develop some gluten, I add the sugar per the recipe's instructions. At this point, it seems that the dough starts taking on a different texture -- smoother, and almost wetter-looking.

What exactly is happening to the dough when sugar is added? Why is the texture changing? Is it really getting wetter? I doubt the sugar turning into a liquid during kneading to cause this effect. I'm assuming something else is going on???
The sugar competes with proteins and starches (read:flour) for water. With less water available for the flour, fewer gluten chains are able to form resulting in a more tender dough.