I liked this one's intro. The way I understand this, is that it is a standard for writing unique things as a smart contract, which allows one to use the EVM to exchange ownership of that unique thing.
ERC721 tokens can be used in any exchange, but their value is a result of the uniqueness and rareness associated with each token.
Basically, with the standard you create a coin which only has one token in it. The standard would then allow anyone to list that item in their Exchange of Stamp Collection, and sell its ownership if the owner approves of it.