Tom
As Jim C says above (and others have too, I'm sure) the answer to your question is "Because they are fun!"
IMHO, being able to design greens (and their surrounds) which are both fascinating to look at and AND playable and multi-pinnable at high speeds (11+), is one of the supreme gifts which defines a great golf course designer. A number of greens at Dornoch are my gold standard for this (e.g., 1, 3, 4, 5, 7, 10, 11, 14, 15, 17) and I remember a few at Pacific Dunes, too.
Rich