If you sell your NFT art for $10 and then someone resells it for $1M, you will get some percentage of the $1M.
We've all heard the stories of "collector buys 10 Jackson Pollock's for $10 in the 70s, sells the collection for $100M today". Wouldn't it be nice if the artist could benefit from that belated appreciation?
Like most other things on the blockchain you don't really need the blockchain to do this, however this is one case where it's actually much simpler to implement the concept using a blockchain and smart contracts rather than any traditional method I'm aware of.