From the API page (http://api.rubyonrails.org/classes/ActiveSupport/Concern.html http://api.rubyonrails.org/classes/ActiveSupport/Concern.htm...) it seems to do

From the API page (http://api.rubyonrails.org/classes/ActiveSupport/Concern.html http://api.rubyonrails.org/classes/ActiveSupport/Concern.htm...) it seems to do two things, 1. resolve module dependencies 2. eliminate some ruby boilerplate in the module code. I kind of liked the idea of extending the instances instead of including in class, to add functionality to instances instead of all objects of the class, but this keeps your model's fat, just the fat is distributed so it's not in the model's single file.