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.
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No, it doesn't, really.
Once it is dissolved, it behaves exactly the same.
Eaters won't know the difference when eating the finished product.
So, tastewise, there is no special reason to use rock sugar.
This answer assumes that you are adding sugar to a polar solvent (water, broth, alcohol...).
It doesn't cover techniques of adding the sugar to oil or other occasions where it is not dissolved.
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In China, highly refined granulated sugar like we find in US is somehow uncommon.
The most common form of sugar is soft white sugar that is less pure and contains a few percent of syrup and other sugar family substance.
These imperfections sometimes affect cooking.
Rock sugar has a relatively larger market in China than the rest of the world for precisely this reason: over there it's the most common highly purified and refined form of sugar, because if it's not pure, it won't crystalize so large.
Crystalization is the further refinement process.
As a result, if a Chinese chef feel the need for highly purified sugar, he/she calls for rock sugar.
This logic is sometimes carried on by chefs that works with granulated sugar regularly that is truly indistinguishable form crushed rock sugar.
So, if your regular sugar is already crystalized high purity granulated sugar, then no need for rock sugar.
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According to this , it is less sweet than regular white sugar which can be used if rock sugar is not available.
Even if it can/ has a brown/golden-ish color, it is not brown sugar which has a more distinct flavor.
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My answer is based on experience;I’m not sure of the underlying science.
Rock sugar is not as sweet as granulated sugar.
It has a broader, less intense sweetness which works well in savory dishes.
It lacks a sweet “punch” which is more suited to desserts.
I use the white kind, though I’m sure
the yellow/brown sort has some additional flavor.
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Why do some Chinese dishes, namely braises and soups often call for rock sugar instead of regular sugar. Does rock sugar have any properties that would make it behave differently than regular granulated sugar?
Why is rock sugar used in Chinese cooking?
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Rock sugar is lower purity and slightly less sweet, but otherwise very similar once dissolved and so you could just use common granulated sugar instead. However, it does have less surface area touchign the oil, which makes it react slower and therfore more suitable in dishes involving coloring the ingredient by caramelization. Chinese recipes use rock sugar because granulated sugar is uncommon in China. Granulated sugar can be used where available. The taste of the end result will be the same. Rock sugar has the advantage, however, that it is less likely to burn in hot oil.
Rock sugar is better for use in hot oil as it is less likely to burn. Rock sugar is less sweet than granulated sugar. It does not change the taste of the finall dish. Granulated sugar is uncommon in China hence they use rock sigar. Rock sugar is not required if granulated sugar is available.