Q. Why should I be using IngramSpark in addition to KDP?
While Lightning Source does charge an annual fee for distribution, IngramSpark - in order to be more competitive with KDP does not.
The main reasons you want to be on IngramSpark:
Keep in mind any discounting Amazon does comes out of their 40% margin and not your royalties. It's better to have your book discounted than not.
You should set your discount with IngramSpark at 55% - that way other retailers will be able to compete with Amazon (since Ingram keeps 15% typically, the net is 40%). There's also an option for 'return & destroy' which I recommend. The 'return and deliver' option can become costly and you'll likely get back books in used condition (since they have been handled in books stores and often damaged in shipment).
You just have to monitor your returns to be sure it doesn't get out of hand. I did have one client who was getting more books returned than he was selling. He wrote a really bad book, but he was good at convincing bookstores to order it.
~ Brian Schwartz