Excellent question! As impossible as it is to explain, I don't mind trying. Depending on when it's enabled and all synced up on one device versus another, it's not necessarily going to restore to that device any stuff that's already been on and deleted from that device even though that stuff is in the iCloud library because said stuff is still on another device!
If you wanted to re-up, i.e. download the entire merged iCloud library onto a device, I imagine you would have to sign out of iMessage on that device and then sign back in and re-enable Messages in iCloud. I've not tried that so I don't know but I wouldn't think twice about trying if that was my goal. Bear in mind that just disabling iMessage on a device doesn't sign you out. In fact, as crazy as it sounds, it needs to be enabled in order to be able to sign out! If you decide to try that and need help, let me know.