With last-gen NFT technology, buying a music video only gets you the music video. If the artist decides to drop an audio-only version of the song later, all they can do is make a new airdrop.
Apply this to enough NFTs and a user's wallet turns into a disjointed and contextually-disconnected mess of airdropped NFTs that have trouble working together, evolving together, and having utility, especially as they individually get sent out into different wallets and scatter further.
With RMRK's Multi-Asset NFTs, one can have a single NFT be multiple forms of media, or even multiple versions of the same type of media. Here are some examples:
An eBook NFT can have one asset that is a PDF file, and one that is an audio file. It can also have an additional one for showing in catalogs, wallets, players - a visual cover.
Once the NFT is loaded into Audible, it just plays - the player knows what media file it needs. If it is loaded into Kindle, the reader knows it's expecting a PDF, so it pull that from this NFT.
If a new version of the audiobook comes out, it is easy to replace the audio asset in a decentralized way rather than make a new airdrop. Same for the PDF version.
A musician can publish a song, and later reward his buyers with a music video of it. Maybe the music video even features all the original buyers' avatars in some way, and is then uploaded as an extra asset to each of the audio-only versions, adding more utility and value to them. The song NFT is now both a music video and an audio-only version, and the NFT owners can switch between the version at will.
Some artists enjoy creating making-of videos of their work. It is not uncommon to see them added alongside NFT paintings, but usually as a separate airdrop or just a video on Youtube. With Multi-Asset NFTs, this too is trivial to add to the actual NFT of the painting. Some artists even add additional extras - like music tracks.
Some NFTs simply have different representations - paintings can have homages, songs can have remixes, etc.
One such example is the Kanaria project. For example, the Super Founder Bird 01 has three assets, a composable NFT, a stylized illustration, and a video of its real-world statue:
This is explained in detail on this page.