iOS6 introduced Shared Photo Streams across iOS devices, and iOS7 gave users the ability to edit those streams on multiple devices.Aside from allowing users to easily share images with friends and family, it's also an easy way to create synced albums of photos across your own iOS devices.
I currently share two Photostreams with myself. One contains a few nice wallpapers, the other contains my own favourite photos. This way I have access to these specific images on any iOS device I own, without resorting to opening Dropbox, emailing them to myself time and time again (the horror) or Flickr/Loom/whatever.
But, the same way you can use these privately shared folders for synced albums, this kind of Photo Stream also allows for quick migration of a Camera Roll from an old iOS device to a new one.
So in the scenario where you bought a new iOS device, didn't restore your iCloud backup (or didn't backup your images) and you want to migrate your Camera Roll from the old iPhone to a new one, you can upload the Camera Roll to a private Photo Stream on the old device. Then, on your new iPhone, you download them from this Transfer-stream back to the camera roll on your new device. Or preferably, just keep them in the Transfer Stream, save you a lot of iCloud backup space.
So.. when is this useful?
- You want to migrate multiple photos from one iPhone to another.
- You want a shared library of wallpapers on all iOS devices
- You want a shared library of favourite photos on all iOS devices
- You want a couple of portfolio folders synced across devices