Imagine you have 3 branches: local-dev-tuneup, bugfix-a, feature-b

Remember in JJ you're always "in a commit", so the equivalent of the git working tree (i.e. unstaged changes in git) is just a new commit, often with no description set yet. (Because in JJ a commit also doesn't need a description/commit message immediately).

So in a mega-merge you can have a working tree that pulls from local-dev-tuneup, bugfix-a, and feature-b, and you can then squash or split changes out of it onto any of those source branches. Like you can avoid serializing those branches before you're ready to.

I've definitely faced the scenario in Git where I have a unmerged changes that I want to use while continuing work on a feature branch. I end up creating a PR for the branch of the first smaller features (e.g. local-dev-tuneup->master), then a second PR pointing at the first (feature-a -> local-dev-tuneup). It works but it's all a bit cumbersome, even more so if feature-a ends up needing to land before local-dev-tuneup. JJ has better tools for handling this.

Or potentially a team member has critical changes with a PR open and you want to start building on top of their changes now. Manageable in Git but you're locked in on a branch of their branch. Now add a second set of critical changes. Can be done in git but you'll be jumping through hoops to make it happen.

Of course you might say that all indicates a workflow/process problem, but my experience is this situations are common enough but not frequent.

(I haven't actually used megamerges myself yet but the article has me ready to try them out!)