Although Granny Smith apples are most often used for baking, I've loved their puckering tartness ever since I was a kid and I still like to just bite into a bright green granny every once in a while.  Tart apples are used in desserts because they tend to hold their texture better and they also add flavor to the dessert besides just sweetness.  Cut up some slices and throw them in a juicer to add a nice tart kick to a summer smoothie.