The thing that makes this so great with these other devices is that you can now say something like "Hey Siri, play Prince in the Den" and music can play over the stereo or home theater system you have connected to your AppleTV.
For me, this means, I can use voice control to send audio to my main music setup or the multiple setups I have around the house connected to an AppleTV or Airport Express.
The Microphones are Underestimated
However, the thing that is often missed is that the HomePod can hear commands in loud rooms much better than any of my Echo devices. If I can listen to music and ask the HomePod to change what is playing in a normal voice without screaming, then I am more likely to use the feature. Of course, with the AppleTV, you get beautiful artwork and/or fantastic 4k screensavers:
iOS and Airplay 2
In the photo below, you can see that I am sending Prince to the Bedroom and Den. AC/DC is playing in the Living Room and Nirvana is playing in the Office.