Yes, they do serve a purpose. They're desinged to keep brush from snagging between the string and belly of the limb on recurve bows. This is not only annoying, but can really wreck/twist/break a recurve if you're not careful. As far as hand shock goes, it's a bit counterintuitive. If you have working recurves or a very short recurved/flipped tip, then yes, this style of nock tends to add a bit of shock. However, if you've got a pronounced static recurve, then no, I've found I get no hand shock (assuming the limbs are tillered/timed properly!). Brush nocks tend to exaggerate any tiller/timing inaccuracies.
The above reasoning works on the much the same principle as a horsebow. Logic would suggest that those long levers (siyahs) would jar your teeth. However, for a number of reasons, horsebows are amonst the smoothest drawing and shooting bows.