"Flatten" comes from the world of desktop image editors. You "flatten" one layer down to the next to make one layer. Sometimes the people who work on these kinds of things forget their users might have never used software with metaphors like this.

I agree that the help page isn't very helpful. I had to reach out internally to understand what it is doing and why it is there.

Hey, I'm the SGEL for User Voice, ping me on slack and I'll be able to help.