- stream from a wide number of subscription services
- stream from your own DLNA server or samba share
- cast from Chromecast or AirPlay
- with multiple devices, you can do multi-room synchronized streaming
- apply parametric EQ, including (in the higher end models) auto room correction
- hook up a digital source (eg, TV) so you can apply EQ
- hook up an analog device (like a turntable) and stream it to the rest of the devices in the house
The app's surprisingly good, the firmware updated regularly.
The cheapest version is under $100. They're like Chromecast Audio's on steroids. When (eventually) they die or get superseded, it's easy to replace without needing to touch the rest of the system.
I have 3 at the moment, the main one hooked up my amp and speakers from the 80s, which I picked up used in the 2000s.