They are used to raise a high voltage, which is then used to accelerate an ionized gas out the thrust nozzle. Heating a gas would also provide for a fast moving reaction mass.
I was just reading Wikipedia’s article about them. It would be great to see a successful design actually working on a rocket. It seems like there are a lot of designs, but even a successful one will still have to deal with government bureaucracy and environmentalists.
I hope Elon Musk will get to launch Starship soon.
Here's a video from Marcus (out of Australia, Tasmania) who follows Space X and other launches on a weekly basis. I watch on YouTube to see what Musk is doing in Boca Chica, Texas and the cape. This is one of his specialty videos about nuclear space propulsion - https://www.youtube.com/watch?v=GLD4DpuGWUA.