One of the most compelling use cases for voice interaction is music. This interaction is compounded when the listener is alone. It’s less jarring to interact with a voice device when there isn’t another party who might think you’re talking to them.
An amazing use case for Alexa would be on mobile device for music control. While this isn’t supported now, I could imagine that might be in the future with Amazon Music Unlimited or a similar paid offering.
EarPods or similar headsets will likely be the next FitBit this year with their major use case being the ability to play music without touching an interface. Perhaps more dedicated hardware could help free up voice from mobile devices for “hearable” music playback.