Why Powdered Sugar Isn't Always An Ideal Sweetener For Warm Beverages - Tasting Table
Granulated sugar doesn't dissolve well in cold drinks, leaving a sweet puddle at the bottom. Using powdered sugar can add graininess due to cornstarch.
In professional kitchens, powdered sugar is often referred to as 10x, referring to the size of the sugar granules (10 times smaller than the granules of regular cane sugar).