The problem is that branches _aren't_ an imaginary concept in git. The easiest way to see this is by looking at the difference between `git checkout master` and `git checkout ae596fe`. The first one checks out a branch, but the second one checks out a headless commit. I can still do many of the same operations in both states (for example, I can create new commits based on the current one), but how the repository changes will differ depending on what I've got checked out.

I agree with you that this is mostly unnecessary complexity — branches being anonymous is one of the big reasons I like jj, and why I think it's so useful to people just getting started with version control, because it is a simpler conceptual model. But it's not correct to say that branches in git are imaginary.