who calls url_for(options=nil), and when?

In ruby 1.9.3 and rails 3.2.6, I built a simplest project:

rails _3.2.6_ new subdomains
rails g scaffold Blog name:string subdomain:string

then, I add a blank function

def url_for(options = nil) {}

into the file app/helper/url_helper.rb

and append a line into app/controller/application_controller.rb

class ApplicationController < ActionController::Base
  include UrlHelper
end

After running, when I click the button Create Blog of the page http://localhost:3000/blogs/new, I alway got the error:

No route matches [POST] "/blogs/new"

I don't know who calls this function url_for with parameter :controller => 'blogs', :action => 'new', when?

I know I must miss some basic knowledge about rails, any advice will be appreciated!