I have a smart tv with various apps, and was wondering what additional functionality do you get from a Amazon firestick?

It rather depends what you get on your TV. which you don't mention

Generally though, Firesticks provide more options than most supposedly 'smart' TV's.

It's portable.

You can load apps on it not available on smart TV's.

If it's just for basic Netflix, Amazon or Catchup use it'll be no different to most Smart TV's.

Will add greater functionality on dumb TV's.

And with Kodi and various add ons sideloaded on to it its fantastic. The only real reason for getting a stick nowadays. Gives great access to film, tv and sport.

Can the firestick be used on a non smart tv?

Yes it can.

Also with the right apps on it such as Kodi works really well. Your TV needs to have a HDMI port.