The Google Home family of devices have built-in speakers of varying quality, but you might have some nicer Bluetooth speakers around the house. Now, you can have Google Home stream audio directly to these devices. After pairing your Home with the speaker, you can control playback and even adjust the volume.
To pair a speaker, open your Home app and head into the device settings. Connecting to a Bluetooth speaker is part of the "preferred speaker" setup (previously spotted on Reddit). That process already existed in the app, but it was limited to Cast-enabled devices. The new bit is Bluetooth support. And this should not be confused with pairing devices with the Home over Bluetooth. That feature rolled out ages ago.
You can add a few Bluetooth speakers to Home and get a multi-room music system in a few taps. When you ask Home to play something, it'll go to the Bluetooth speakers instead of using the internal one. You can also say things like "turn up the volume" to control the Bluetooth device. This works with all versions of the Google Home, too. This feature is still rolling out, so it might not be live for everyone yet.
Google has added details to the support page. It includes the steps to pair, manage, and re-connect to BT speakers.