The reason to use whole spices instead (presumably) of powdered/ground spices is that with more than one constituent you've no way of knowing how much of each you might be getting. So when some mixes like an apple pie spice mix are used, from say 1 teaspoon you might get 10 cloves worth with a quarter of it being nutmeg and only the tiniest amount of cinnamon.
What's wrong with that you might ask, cloves are quite powerful flavouring so that amount would likely make a gallon undrinkable.
Ergo, its about having full control over the levels of each flavour.