If, for example, it was set so that types of stone that would normally be able to contain a certain resource(Just as an example, Malachite from Marble), and you have found a Marble biome but not a vein of Malachite, you could collect a bunch of Marble stone or gravel or sand and sift through it to find the items.
It's much less random than relying on mob drops, because if you assign a material to the mob you have to: 1) hope the mob spawns, 2) be able to find it, 3) hope it drops, and 4) repeat until you're finished.
Using Ex Nihilo to fill in the gaps, you only have to: 1) find the stone-type you want, 2) sift materials. Yes, it is also reliant on RNG, but unless it's set to something absurd(1/1000 or something chance to get an item), you will eventually get what you want. It's 'guaranteed' RNG rather than 'may never happen' RNG like relying on mob spawns and drops.