You don’t need to root it to install other, more useful apps.
For example you can run your own Plex server on a spare PC, download movies and TV shows (I hear you can use torrents for this), and install the Plex app on your TV stick to watch them on the TV. Or, ask a friend for access to their Plex server :)
If you like cable TV, you can sign up to a much cheaper IPTV service and use the TiviMate app.
On the Onn one, I think you can just use Google Play to install other apps. On the Amazon Fire one, if it’s not in Amazon’s app store, you’ll have to download the APK file.