South: Why creating a new empty migration when pulling in other developer's model changes?
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Below is extracted from the section of the official South documentation on [Team Workflow][1],

> The second thing to note is that, when you pull in someone else’s model changes complete with their own migration, you’ll need to make a new empty migration that has the changes from both branches of development frozen in (if you’ve used mercurial, this is equivalent to a merge commit).

I don't see why the need for creating a new empty migration in this case. Shouldn't the developer just run `./manage.py migrate` after pulling in the model changes (and the corresponding migrations) from others? What am I missing here? 
 


  [1]: http://south.readthedocs.org/en/latest/tutorial/part5.html#team-workflow