The sugar will dissolve a lot faster when you boil it. I don't think there are other differences. But, simple syrup will dissolve in cold drinks, while it will take a long time for sugar to do so. If you are adjusting the lemonade to your own taste, you can start with less simple syrup than called for and slowly add enough until you like it. You can't really do that with sugar granules. I almost always have simple syrup on hand. It lasts a long time.
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Robert A. Heinlein