ORIGINAL: KodiakArcher
They are mostly cosmetic but they have some function as well. They make it easier to find a lost arrow, they can be used to stiffen arrow spine reaction (by putting more weight on the tail of the shaft), they add total weight to the arrow and they increase fletching adhesion. You can use either the standard ones or the special carbon ones on your carbon arrows.
I agree they add some weight, although very little, but I am not sure about stiffening the spine. Anyone else?