Why is rock sugar used in Chinese cooking?
while all of the above answers are correct, I want to provide a perspective as a native Chinese. Rock sugar is better used (than granulated sugar) when you try to make dishes involving coloring the ingredient (by caramelization, dishes like braised pork belly (Hong Shao Rou need this step) mostly because of the shape difference. Rock sugar has less surface area touching the oil while granulated sugar immerses itself in, which makes it react faster, and more likely to get burnt.