What we're trying to do here is really concentrate the apple flavor. I experimented to try to change up this recipe a bit from my usual apple cider donut recipe and found that this really does boost the apple flavor. If you're trying to speed up the amount of time it takes to make these donuts, you can just use ½ cup of apple cider and skip the reduction step. However, I recommend you really give it a try because it gives a bolder apple flavor to the donuts.