.js.erb files are for controller actions, such as create, when you want javascript to be executed when the action completes. For example, when you want to submit a form via ajax and want display an alert when everything is done, you would put the
$('#business_submit').click(...)
in your application.js or *.html.erb, and your create.js.erb could look like this:
alert('New object id: ' + <%= new_object.id %>);
This way, the javascript that is getting returned to the browser can be first rendered by Erb, allowing you to insert custom strings/variables/etc just like you can in .html.erb templates.