Most NFT projects today are only mock-scarce. Sure, there is a limited supply of them, but the utility of these NFTs (if any) is uncapped.
As an example, you can log into 500 different instances of Sandbox with the same wallet using the same NFT. You can equip the same hat onto 500 different in-game avatars at the same time, because its visual representation is just a client-side thing. The platforms and projects are not at all Sybil-resistant, and the NFTs not scarce.
With RMRK, and to use games as an example again for simplicity, a piece of metaverse land IS an NFT. And for an avatar to be IN that land, it has to be IN that land - literally sent into the other NFT. By definition, if it is in one land, it cannot be in another. If a hat is equipped on one avatar (by being sent into it and then equipped), it cannot be on another.
With RMRK's standards, true and provable digital scarcity as originally promised by NFT technology comes default.