Imagine you have a table that has 5 columns, and you need to split this into two tables, because in your iterative process, your application has increased in complexity.
Should migrations change the data, as well? After all, you are dropping two columns and moving them to another table, and these two tables now probably need a foreign key to connect them, too.
What has me confused here is that as far as I know, migrations are only responsible for changing the structure of the database. It is all fine on a development database, but what would you do when you want to push this onto a live database?