Some of these things may or may not apply to you but keep in mind they are features many users want/need.
1. App store - for more than just streaming services like games, shopping apps, security cameras, etc etc....
2. HomeKit Hub
3. AirPlay
4. Preferred UI
5. Use on TV's that have less desirable smart features
6. iTunes home sharing
7. Easy integration into other Apple services like Apple Music
8. iCloud app syncing (downloading an app your an iOS devices automatically downloads onto the AppleTV).
etc.
Again keep in mind some/all of those may not be features you want or need however they are features/function available with an AppleTV that a SmartTV may not provide.
1. App store - for more than just streaming services like games, shopping apps, security cameras, etc etc....
2. HomeKit Hub
3. AirPlay
4. Preferred UI
5. Use on TV's that have less desirable smart features
6. iTunes home sharing
7. Easy integration into other Apple services like Apple Music
8. iCloud app syncing (downloading an app your an iOS devices automatically downloads onto the AppleTV).
etc.
Again keep in mind some/all of those may not be features you want or need however they are features/function available with an AppleTV that a SmartTV may not provide.