Google is slowly fleshing out the abilities of Google Assistant across multiple platforms. The most recent update gives Google Assistant on your phone the ability to control Chromecast.
The feature was first spotted by Android Police. Google Assistant on Google Home already had the ability to control Chromecast through voice dictation, but the feature was missing from the Google Assistant version on your phone. Why were things like that? We have no idea, but Google has seen the error in its way and pushed out the very useful update.
It’s a great move on Google’s part to reduce the awkward fragmentation its Google Assistant service was experiencing across multiple platforms. The update will work with the regular Chromecast, Chromecast Audio and other devices that have Chromecast built in.
Google is rolling out the update sporadically. You may have to wait a while for your device the receive the update, but it is on the way.