This is in the special case where you want the application itself to determine the initial state of the volume. For example, the developer might want the database pre-loaded with a certain schema, default data and so on. External volumes/bind mounts are a way for the sysadmin to override how and where the contents of a volume are stored.

Re: documentation, you can take a look at this example: http://docs.docker.io/en/latest/examples/couchdb_data_volume...

I hope this helps! Feel free to join the IRC channel (#docker on freenode), it's unusually active and helpful, I'm very proud of how nice the people are on there :)