Allowed for posting of articles

A visitor to the site can add an article with a required title field and a URL and/or content field. Currently very basic (read: ugly) and can allow the posting of an "article" with only a title and no URL or content.
I think Rails will automatically assume the `create` action if the instance isn't persisted (which in this case, it isn't).  Any specific reason why the `url` is specified here?