This is an example of Ownership of Experience.
With last-gen NFT technology, replacing an NFT to evolve or change it requires either centralized metadata, or a burn-and-mint mechanic.
With RMRK's Multi-Asset NFTs, this is easier.
If an asset is added to an NFT with an extra argument provided targeting an existing asset by asset ID, that targeted asset will be replaced.
This makes for good evolution mechanics where an NFT can grow or change over time. E.g. a tree that grows from seed, to sapling, to young tree, to mature tree. Additionally, assuming the tree NFT needed to be watered with 💦 emoji every week or else it would die, it is possible that yet another asset could represent a tree's death stage.
Each stage of its growth is a new asset, replacing the previous one.
This is also useful in games. Imagine Pikachu evolving into Raichu by just having a asset replaced in a fully decentralized way, no single point of failure in between.