What does this line do?
The above line adds the database 'qonos' to the repository 'Qonos Migrations' with the current version set to 0. When run against an existing database (whose version is equivalent to 6), the migration tries to run the scripts starting from 001 as the current version is set to 0. Since the existing db has all the relevant tables (until 006), it throws an error saying, 'table already exists'.
But when I run 'version_control' before 'db_sync' on an existing db,  the db 'qonos' is added to the migration repository 'Qonos Migrations' with current version set to 6. Hence when running the 'db_sync' scripts from 007 will be run against the database and that should ideally succeed.

If you have to run the version_control command on a fresh db before running db_sync, does this line accomplish anything?
Yes, this line will throw 'DatabaseAlreadyControlledError'. Reason being, when the 'version_control' on a fresh db is run, the db is added to the repository and the version is set to 6 as the migration tools finds the latest version of the script to be 006. And when the 'db_sync' is run later, the above line will find that the db is already version controlled and hence will throw database already controlled error.

This was an excellent question Alex. Thanks for asking me.