A problem I run into when working with other people is that code reviews take forever and I need to build on top of them. Code gets merged while it's being reviewed, and it becomes a burden to keep rebasing your stack of PRs. It's also difficult to do things like designing each PR against the main branch, but testing all 3 of them together. (Sometimes you want to write the docs / take screenshots as though all your features are merged as-is.) jj makes all this trivial. You tell it what you want and it does it without involving an index or working copy or interrupting you to resolve conflicts.
I've found that it really makes me less annoyed when working with other people. I don't know why it takes people longer to review code (or to even open the review request) than it takes me to write things. But it does, and jj is what keeps me sane.
To be fair, I also use it on personal projects because sometimes you have 3 things you want to try at once and they're not related to each other. Upstream isn't going to change without your understanding, but it's still mechanically something to maintain the rebases on those 3 branches. jj just makes this burden go away.
Having said that, I don't know why a "jjhub" is needed. Github seems fine. jj's just a UI for git.