One the beautiful things about PoW is that it allows for near perfect collaboration without any coordinations.

For example if you’re spending BTC, then there are a number of people around the world who don’t know each other or you — all collaborating to get your message (your transaction) broadcast to the entire word now and forever.

In a messaging system, using PoW means millions of people who don’t even know each other exists can easily collaborate to spread a certain information.

That’s true even if each of them don’t have the resources for publishing one message.

Payments don’t allow that in the same way. Unless you sell lottery tickets for publishing messages — which is sort of the same but with a trusted third party.