easier collaboration with others that you need privacy on. A backup of your repos(though you should have backups on this too). a canonical version of your repository with read access for your team but commit access for only a select few.
I am sure there are more scenarios but this is just what I could think of in a min.
I am using git because I hate svn (so I use git-svn), so I am curious how a pure git setup would work...