To track a remote SVN repository. To edit commits earlier in history when you need to correct minor bugs/CR comments/commit descriptions. Edited to add: squashing lots of tiny safety commits into a commit with an entire feature in it.
The svn issue seems under-appreciated. In my experience, far and away, the best way to merge subversion branches is to pull them in with git-svn, rebase, and push back to subversion. I've done this more than once for projects that I'm only tangentially involved with.
