Apple has released a new version of macOS High Sierra that brings Messages in iCloud to the Mac. macOS 10.13.5 is now available for all compatible Macs through the Mac App Store.
Messages in iCloud can be turned on from the preferences section of the Messages app: Preferences → Accounts → Enable Messages in iCloud.
Once enabled, Messages in iCloud has a few benefits. If you delete a message from your Mac, it will disappear from all Macs, iPhones, and iPads running the latest software update. The other benefit is all message history will appear when setting up a new Mac from scratch. This previously required restoring your Mac from a backup.
macOS 10.13.5 is available for all Macs running macOS High Sierra from the Mac App Store