As you can see, Shouse's sinker dives in to lefties while his slider is a nice complement, breaking away. Neither is thrown exceptionally hard.
Both of Nelson's main pitches break in to righties, and they have to look remarkably alike to the hitter, causing confusion.
So Cormier added that splitter, and saw an immediate step-up in performance. Probably because he used it in place of his fastball, which looks awfully straight.