What does iCloud sync do in Messages?

What does iCloud sync in Messages do? I'd have thought that it was a way of ensuring that messages on one of my devises appear on the others, but that's not how it works. For example if I have Messages off for a bit on my Mac, none of the messages sent or received on my other devices ever show up in Messages on my Mac. So "sync" must be doing something else. What is that and what is accomplished by enabling enabling it? What would happen if I disabled it?