Probably to take a series of commits and decide "this one goes on branch A, this one on branch B", e.g. if you intermingled fixing bug A and B in the same branch, you could more easily go through and assign each commit to a new branch.

The existing workflow for that would be (there are several possible workflows, but this is what I would do):

  git checkout intermingled-branch
  git branch bugfix-A
  git branch bugfix-B
  git checkout bugfix-A
  git rebase -i
  # Edit the file, keep commits that fix bug A, drop commits that fix bug B
  git push origin bugfix-A:bugfix-A
  git checkout bugfix-B
  git rebase -i
  # Edit the file, keep commits that fix bug B, drop commits that fix bug A
  git push origin bugfix-B:bugfix-B

Fun fact, you can also automate more of that existing workflow with the rebase interactive TODO list. Roughly something like:

    pick $ACOMMIT1
    edit $ABMIXEDCOMMIT # split out A parts
    pick $ACOMMIT2
    exec make test # test everything compiles
    update-ref bugfix/A
    exec git switch --detach main # new branch point
    pick $BCOMMIT1
    edit $ABMIXEDCOMMIT # split out B parts
    pick $BCOMMIT2
    exec make test
    update-ref bugfix/B
    exec git merge bugfix/A
    pick $FOLLOWUPCOMMIT
    exec make test
    update-ref intermingled-branch # or bugfix/C

It's been a while since I've attempted a rebase TODO list that wild, so I may have forgotten something, but rebase interactive is pretty wild what it will let you try to automate.