When it's owned by an external party, you might not be able to (easily) trade it. When it's decentralized, nobody can stop you from trading it.
Yet your two sentence explanation is the most comprehensive.
NFTs enable the possibility of a market that is orthogonal to the channel that distributed the token.
The viability or existence of such a market is not guaranteed, but the likelihood that one could emerge at all has gone up thanks to the novel properties of NFTs.