1. Work on smaller projects with simple histories. Git is perfectly satisfactory.
2. Work on huge projects with very complex git histories and weird legacy workflows (primarily Linux kernel).
jj is only interesting for part 2, but then it's only useful if it's easy to bridge into the Git world that everyone else uses.
But this workflow is exactly the kind of thing that seems handy. There's so much stuff that Git can totally _do_ but could just do a lot _better_.