What is the --rm flag doing

Question

I'm new to Docker and don't yet have a mental representation of how it works. I am a complete novice.

The --rm flag has been included in all of the samples I've looked at, such as

docker run -it --rm ...
docker container run -it --rm ...

Why does the --rm flag appear in these commands? Why would I remove a container that I had gone to the trouble of setting up or downloading with the nice content in it? I'd like to save it in case I need it again.