Calcium. The same reason you add egg shells to potting/gardening soil. But the ph is the target as mentioned. Check out Yoopers question on simple PH for brewers in the brewing science section. It'll blow your mind. Lol
Calcium might be true. But I wonder how much you have to use for enough calcium to leach out. I know with gardening, the effects are more long-term. Eggshells take awhile to decompose and offer up that calcium.
Probably acts like rice hulls in a way too when it's time to sparge.