Memoji are the new personalized avatars introduced in iOS 12. They're based on the appearance of the user and you can customize and tweak different features. They're only supported on the iPhone right now, though. The major reason they'd need to transfer via iCloud is if an iPad that could also use Animoji is coming.
There have been rumors floating around that Apple will introduce a new iPhone X-style iPad Pro sometime this year. Code within the iOS 12 developer beta showed a bezel-less iPad with no Home button. Some assumed it would be revealed at the September iPhone event, but others anticipate that Apple will do another computer and iPad combined event in October. Apple Insider points out that code within the iOS 12.1 beta also discusses "iPad2018Fall," which is another sign that there is indeed a new iPad on the way this year.