In short, they can hold a lot more heat than the stones, mimicking the effect of professional pizza ovens.
So the idea is to cook a pizza in the shortest possible time in order to thoroughly cook the dough and outer layers, whilst leaving the ingredients with a delightful freshness. With a conventional oven the process takes much longer, tending to cook everything evenly, producing a drier pizza in which you don’t get that ‘brick oven effect.’
I’ve tried the stones myself, heating them on max heat for a whole hour beforehand. They can help a bit, but it’s still not the same. All that’s my take on the situation, anyway.
I checked the FV just now and I don’t see a pizza community here that goes in to this stuff. Unfortunately for now, you’ll have to visit the evil empire for more precise info.