I read through Amazon’s web page with the details for this. It says that in the short term they want the WiFi sharing for devices like webcams and echo. Longer term, they plan to introduce location tags (sounded like the tile tags) so that you could find items. I’d imagine the shared WiFi would not only help you find your keys in your house, but also if you dropped them somewhere during a long walk.
I opted out. They need to start paying me if they want to use my overpriced xfinity data capped service.