I think there's a sense that magit and jj are in some way equivalent tools, although I don't have enough experience with magit to be sure. They both sit in top of git and expose the underlying model of git far more cleanly and efficiently than the standard git cli. The difference is that magit uses interactivity to make git operations clearer, whereas jj tries to expose a cleaner model directly.

That said, there are additional features in jj that I believe aren't possible in magit (such as evolog/interdiffing, or checked-in conflicts), whereas magit-like UIs exist for jj.

You want the history of a specific commit because if you, say, fixup that commit, you want to know how the commit has changed exactly over time. This is especially useful for code review. Let's say you've got a PR containing a refactor commit and a fix commit. You get a review the says you should consider changing the refactor slightly, so you make that change and squash it into the existing refactor commit. You then push the result - how can the reviewer see only the changes you've made to only the refactor commit? That is an interdiff.

In this case, because you've not added any new commits, it's trivial to figure out which commit in the old branch maps to which commit in the new, fixed branch. But this isn't possible in general (consider adding new commits, or reordering something, or updating the commit message somewhere). In jj, each commit also has a change ID, and if multiple commits share the same change ID, then they must be different versions of the same changeset.

You want the history of the repository which includes the history of each commit, because it's a lot easier to type `jj undo` to revert an operation you just did than it is to find the old state of the repository in the reflog and revert to it, including updating all the branch references to point at their original locations. The op log in jj truly is the ultimate undo tool - it contains every state the repository has every been in, including changes to tags and branches that aren't recorded in the reflog, and is much easier to navigate. It is strictly more powerful than the reflog, while being simpler to understand.