But they aren't actually buying the art, are they?
They are buying a slot in the ledger that says they own a token. The token just has a copy of the art embedded in it.

It sounds like buying 1 of 500,00 Nike Air Dunks. But the one you buy has a copy of "Michael Jordan" signature stamped on it by the factory press and paying $50k for it.

All of the other "Nike Air Dunks"(NFT) sell for $40. A $40.00 shoe(NFT) with a stamp.

I don't get the motivation because you can just buy another NFT from the same blockchain and download a copy of the picture from online.

Unless some type of art ownership rights transfer with the token, what is the motivation?