One of the main benefits to Cast-powered speakers is they can be added into groups, using the Google Home app. This way, you can have the same music playing across multiple rooms without any latency. The feature used to be limited to Google Homes, Chromecast Audio units, and other Cast-enabled speakers — but now regular Chromecasts can be added as well.
Thanks to a recent firmware update, you can now add video Chromecasts to speaker groups. This has been a long-requested feature, so it's nice to see it finally arrive. If it's not working for you yet, give it a few days — the rollout might not be finished yet.
Google has now updated its speaker group support page to formally confirm this feature's availability. While groups are now supported on both Chromecast and Chromecast Ultra units, that doesn't extend to 1st-generation Chromecast models, which are left out from this upgrade.