For me it's more about integration with internal services.
I agree with you, for normal chat messages such long messages do not seem necessary but for hooks I can see the odd occasion when it might be of use when integrating with our local build/deployment systems. I would hope that generally people would use them in such a way that they aren't getting 4000+ characters of data but it's certainly a possibility in certain limited circumstances. At the moment the return message from the outgoing hook call would be rejected and not shown if more than 4000 characters are returned.
I could be wrong but from reading the documentation, slack hooks and messages don't have a character limit that I could see which would mean that the idea of being compatible with them whilst essentially is true, isn't technically 100% in edge cases.