To get sugar from plant form to powder form takes much water. Water naturally dissolves sucrose, even when it is still in the plant. boiling, either by turning up the heat or exposing the water to a vacuum turns sucrose into molasses. When it cools and is centrifuged the molasses turns into brown sugar crystals, because the pure crystals will still be covered with a thin layer of molasses.
Brown sugar, with its tiny bit of syrup has only slightly less
concentrated sweetness, and 0.23 fewer calories per gram, than white
sugar. This layer can be boiled farther and centrifuged again until
it becomes naturally white sucrose crystals.
Cookies made with all white sugar spread wider. Cookies made with
brown sugar do not spread as much. The hygroscopic brown sugar
absorbs more liquid and keeps dough together.