What purpose does simple syrup serve? I read that you put it on prior to frosting and/or applying fondant. 
It makes a moister cake. Most mix cakes won't need this unless they have been over baked, but some scratch cakes can be a little dry and this will take that away. Of coarse it tastes good and adds flavor to any cake. HTH
Cake mixes have preservatives in them so you can make them days ahead and not have the cake dry out. ( It gives you extra time for decorating)
Scratch cakes don't have the chemicals in them, and so can become dry over the 2-3 day process of decorating an elaborate cake. The simple syrup seals the cake and helps to retain moisture. It is also an opportunity to add a layer of flavor to the cake.
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