They're not really comparable to Bluetooth receivers. The killer feature is synchronized multi-room audio without paying exorbitant prices for proprietary speakers (Sonos I'm looking at you...). I have them hooked up to powered speakers (PC 2.1 style) in the bedroom and bathroom, a mini-stereo on the deck, a ghetto blaster in my workshop and my media room AV amplifier. For parties it's fantastic to have seamless audio around the whole house.
I have to wonder why Google did this. It seems like a strategic move to push people towards their "smart" speakers. Personally I really don't want something with a microphone in it, just good quality sound at a good price. Very sad.
By the way, it's easy to cast audio only from YouTube or any other app - just go to the Google Home app and cast your phone's audio to the device or group.