It needs the internet for most of your needs. Personally, it is your phone that is converting to TV.
1. Phone OS updates are much faster than Android TV updates
My phone has Android 10 and TV has Android 9. I can make use of Android 10 apps for use with the TV ( if it is compatible )
2. You get a wider catalogue of apps
ie, you can use apps that support Chromecast by using the cast button. If it does not support casting, you can mirror your phone using Google Home app.
3. Video calling
It is easier to mirror the input video on screen and have a better calling experience.
4. Privacy
I do not trust FireStick or other devices listening to us all the time. Yes, our phone might be listening, but I don't want to extend it to the voice remote as well.
5. Gaming
You don't have to rely on your TV's weak processor or RAM. Just mirror the screen and enjoy widescreen gaming.