When running the mongo shell and providing a .js file to run, it would be convenient if the user did not have to specify a host on the command line as well.

Lets say I have a script which connects to a certain host/db and then does some actions against that connection. If I don't have a mongo instance running locally and try to run the script using the shell:

mongo <my script>.js

It will fail because it can't connect to localhost. ideally, it should not try to connect to localhost, but just be able to run my script (which itself specifies where to connect to). This is kind of a special case and basically allows you to start the shell without an initial connection.