A data migration tool, what it really means is if you change the data model and want to keep the data in there, you will need this app. At first I thought it's tedious for early stage development when data isn't important (why not just syncdb??), but once I realised it's actually quicker to use south through out the development cycle.
e.g you added a field, just run
python manage.py schemamigration app_name --auto
python manage.py migrate app_name