There are tons of use cases for AR:
- https://storify.com/lukew/what-would-augment-reality
- http://www.madewitharkit.com/ideas and their twitter https://twitter.com/madewitharkit
Headsets have a lot to do with AR, which is why HoloLens is a headset; headsets let it be handsfree, use the entire circumambient space, and provide an important and intuitive control for the portion of reality the user is interested in having augmented.
They also allow for stereoscopic 3d, which is useful, though not always essential, for AR.
Can't see any of them being worth putting a headset on.
Can't see any of them being worth launching an app on a phone to stare through a camera at.
Some of them need some pretty next level ai.
The rest are pretty underwhelming. Lots of things that make great demos, but not many that feel like I would come back to them.