I wrote a very popular tutorial for jj, and I didn't use workspaces until a few weeks ago. They're useful for the same reasons git worktrees are: most recently, people use them for doing work with multiple AI agents in parallel, but historically, I've seen people use them for things like "this project's build takes 30 minutes so I want to work on something else while I wait for that".

> I don't see how creating a worktree in a new folder and opening a new editor is more convenient than creating a branch at a certain commit.

Worktrees are about being able to work on multiple branches at the same time, fundamentally. When you want to be doing something on one branch and something else on another branch simultaneously.

That does mean it's, IMHO, a fairly niche feature.