Purely local branches are where you do your own development. You may use purely local branches because you don’t have commit rights to the Savannah repo. You may also use them if you are doing some work that isn’t ready for sharing with the rest of the team, or cannot be committed for some other reason.
For example, for around a nine-month period, the maintainer kept a purely local branch for some contributed changes for which paperwork had not yet been completed.