The iOS 11.4 update added Messages in iCloud and AirPlay 2, but it also brought issues along with it. The iOS 11.4.1 update keeps these long anticipated features and includes fixes for some iOS 11.4 problems.
Specifically the iOS 11.4.1 update;
Fixes an issue that prevented some users from viewing the last known location of their AirPods in Find My iPhone.
Improves reliability of syncing mail, contacts and notes with Exchange accounts.
This update also includes other bug fixes and security upgrades. It's possible that this update includes a fix for bad iOS 11.4 battery life.
Messages in the Cloud allows you to backup your iMessage conversations to the cloud just like you back up your notes, contacts and photos. This means they are backed up in case something happens to your phone, but that's not all.
Messages in the Cloud syncs deleted messages and threads across your iCloud devices. This means everything will show up the same across your Apple devices. With this in place you should see the same conversations everywhere, all of the time.
This also backs up the photos and attachments in the cloud so you have more room on your iPhone or iPad. When you get a new device all of the messages sync to that device so you don't lose anything.