Yes, because you're crushing up the cells and releasing the stuff our puny human guts can digest. Raw kale is mostly undigestible by humans unless you're willing to sit and chew on every leaf for a few minutes like a cow.

Unfortunately I'm having trouble finding the sources on this. All of this breathless superfood wankery is really polluting the search results.

http://healthyeating.sfgate.com/digestion-kale-11913.html

http://gi-north.com/kale-good-for-you-and-your-digestive-tra...