Ingram is a distributor. All books from traditional publishers go through Ingram. Go into any Barnes and Noble and ask them who distributes their books, they'll tell you Ingram. Stores don't by books directly from publishers, they buy books from Ingram. The publishers pay the printing costs, but Ingram handles everything else.
IngramSpark piggybacks onto Ingram.
Bookstores use Ingram because they've been around forever, are trusted, and best of all, have an established history. If I owned a bookstore, I wouldn't use anyone else, but Ingram. IngramSparks gives you, as a publisher, credibility. They know the quality of the books won't be an issue and that if they sit on their shelves for too longer, they can return them. They also know they can place an order and get more within X days. Sure, they could do it with Amazon, but they already have an account with Ingram and are already ordering all their books from them.
I have a book on IS that was picked up by brick and mortar stores. The only reason I know this is the stores went through the ISBN and contacted the publisher requesting more books in the series. If I had used Amazon instead, those stores never would have seen the book in the catalog and never would have taken a chance on it.