Another example is that there is this simple monorepo that holds both the latest _BlaJS_ frontend and the backend API.

You are working on stuff in the backend, but it sure would be nice to see it in the frontend so you jury rig something in the frontend to display your work as well as some console.log() commands. Then you forget to revert changes to the frontend before pushing the branch.

In jj you would start with these as separate branches. Then you work on a merge of these. Then you use the absorb command to auto-move the code you are working on to the correct branch or you squash the changed files to the branch. Then you can push the backend branch to server and PR it. Then you throw away the frontend branch or just leave it there so you can use it in a future feature.

A real case from my work. I had to work on an old Python project that used Poetry and some other stuff that was just not working correctly on my computer. I did not want to toucj the CD/CI pipeline by switching fully to uv. But I created a special uv branch that moved my local setup to uv. Then went back up the tree to main and created a feature branch from there. Merged them together and worked out from that branch moving all the real changes to the feature branch. Now whenever I enter that project I have this uv branch that I can merge in with all the feature branches to work on them.