Its meant to save you a step. Before at whole foods you had to get out your phone, open the amazon app, scan your prime QR code, then get a card and pay. This just does all that with an enrolled palm.
I still don't trust it. I laughed at it when I saw it and even the clerk admitted it was dumb.
AFAIK an unlocked phone can hotspot no matter what. My plan does not include mobile hotspot data and yet I use it all the time.