The Messages app on your iPhone can quickly grow to be one of your biggest storage hogs, especially if your friends use it to share a lot of photos with you. If after taking a look at Settings -> General -> iPhone Storage you decide you want to start deleting message threads to free up storage, you probably will want to keep some of the images in those threads.
While you would always get your messages on all of your iOS devices, a great annoyance was that if you deleted one on one device, it would not be deleted on your others. Syncing Messages to iCloud not only solves this, but it saves space on your device as well by storing all but your most recent threads in your Cloud. To activate it, head to Settings -> Messages and toggle on Messages on iCloud. When the "Sync Now" button turns blue (it could take a few moments), tap it to sync your messages.
Bitmoji is an app that allows you to create "your own personal emoji" and use it in a variety of messaging apps. Bitmoji was originally owned by Bitstrips, which was acquired by Snap Inc. (parent of Snapchat) in mid 2016. While it was quickly optimized and integrated with Snapchat, it can be used with other messaging apps, including iMessage. Here's how to install the Bitmoji keyboard on iPhone and use it in iMessage:
Adding the Bitmoji keyboard
Many apps that you downloaded from the App Store are compatible with the Messages app. For example, if you have Words with Friends, you can play that against a friend within the Messages app. To enable it you have to go to the iMessage App Store by opening any conversation in Messages and tapping the "A" button to the left of the text box. Then tap the four oval icon in the bottom left, then the blue "+" Store button. Now tap the "Manage" tab and you will see all of your apps that are compatible with Messages. Toggle on the ones you want to use.
To get to the iMessage App Store, open up the Messages app and open a conversation. To the left of the text box you will see and icon that resembles an "A." Tap on that, then tap on the four oval icon in the bottom left. Now you will see a blue "+" icon, which is the store. Tap on it to go to the store.