There are several iOS 8 features that are limited to the iPhone 6 Plus only. Since the display is larger, items such as home screen rotation do not appear on other devices running iOS 8. In another example, each conversation in the Messages app is labeled with photos of the contacts participating in the thread.
These pictures are sourced from the Contacts photos stored on the iPhone. Now with ConvoPics, any device running iOS 8 can activate these conversation pictures, no iPhone 6 Plus required. The feature makes it even more convenient to locate conversations in the Messages app, by scanning for the person's face.