Linked from
The 6 pages that link to Simple syrup, each with the reason it gives.
CocktailRelated: It dissolves readily and gives cocktails controlled sweetness.
Iced teaRelated: It dissolves readily in cold tea, unlike granulated sugar.
Cream sodaRelated: Soda fountains used sweet syrups as a base for mixing cream soda.
SangriaRelated: It adds sweetness without leaving undissolved sugar in chilled sangria.