Basically they are Android computers. Some just run stock Android, some have the Google TV launcher as a simplified front end.
You can use one on a network to stream content you have (for example, files from your own DVDs that you ripped) to your TV. That's basically what I use mine for. I have a library of ripped DVDs on a home server (Windows Home Server because I'm old school). My Sony TV can stream them (iy runs Android) using VLC. The Vizio 'smart TV' in the bedroom can't, so I use an Android TV box (ABOX) to stream them (HDMI to TV).
- 2017 newest Amlogic S912 Octa-core android TV box: Rapid, stable configuration coupled with the highly customized Android 6.0 System
- 2GB RAM & 16GB ROM, It provides plentiful room to quickly start your apps and games with expandable SD storage up to 32GB, it also greatly improves the reliability of your TV box and prevents crashes due to lack of memory
- Awesome CPU & GPU: Amlogic S912 Octa-core 64-bit ARM CortexTM-A53 up to 2GHz, plus 750MHz+ ARM Mali-820MP3 GPU processor. Its high machine configuration gives you ultra-fast running speed and professional image processing ability
I used to use Kodi to do that. Kodi is another story. There are legitimate and illegitimate Kodi addons that let you stream content - some legally. With Android 6 it's easier to just use VLC and access my video library.
I can use all most all of the same legitimate Google Play Store Android apps you'd use to watch things like HBO (HBOGO), etc. but the Spectrum TV app refuses to run on an Android device with a separate screen (runs fine on my Sony TV) - which is because the cable companies want you to rent cable boxes (I don't, I run a 6-tuner networked cable card box controlled by Windows Media Center on a PC with WMC extenders at each TV - and ROKUs). It runs Netflix and Amazon Prime Video apps, etc.
Basically one of these turns your non-smart TV into a smart TV or adds real functionality to many proprietary system 'smart' TVs (especially older ones).
I put a Google TV-like tile app launcher on it.