The crypto NFT's
Because a lot of buildings are made of stone, cement and dust, to build those NFT’s which upgrade your buildings will require stone too.
And not only buildings but various quests and events will require some stone.
Your buildings will require in time various reparations too, otherwise new NFT’s wont be crafted. However the decay is not designed to be an annoying thing (for example repairing your base each few hours), but just another mechanic of the game.
if in the future we see that the stone is not used, we could make it part of the after-PvP base repairs.
From second update when we will diversify the items and blueprints, from the stone quarry you will be able to gather some different resources too, like sand, which will be required for various recipes.